feat(conformance): add WP-18 corpus-integrity gate + fix 6 mislabeled gf16 rows

Adds a stdlib-only conformance-corpus integrity gate (sibling to the catalog
count-gate). Six independent checks: pack-set == SSOT (83), INDEX recount,
per-pack SHA freshness, finite-row abs_error re-derivation (TestFloat-style
independent oracle), special-value same-class round-trip (nan/inf), and an
auditable honesty allow-list for disclosed nonzero errors.

Self-test (15/15, every check FAIL-reachable) gates the gate before it runs.

F-1 correction: the re-derivation oracle exposed 6 gf16 rows storing
abs_error=0.0 while decoding off-grid (phi, inv_phi, phi_squared, inv_phi_sq,
e, pi). Round-trip was correct; only the error label was wrong. Each abs_error
is set to the measured value and allow-listed as not_on_grid. The anchor
identity phi^2 + 1/phi^2 = 3 remains exact in f64. Verdict now CLEAN.

- tools/wp18_conformance_gate.py, tools/wp18_selftest_gate.py
- .github/workflows/conformance-integrity-gate.yml (self-test then live gate)
- conformance/vectors/abs_error_allowlist.json (8 disclosed entries)
- conformance/integrity/fixtures/ (frozen evidence rows)

Closes #1079

Author: gHashTag

.github/workflows/conformance-integrity-gate.yml

@@ -0,0 +1,63 @@
+# WP-18 conformance-corpus integrity gate (CI).
+#
+# Sibling to catalog-count-gate.yml: that gate locks the SSOT catalog COUNT (==83);
+# this gate locks the conformance CORPUS against silent drift and undisclosed error.
+# Runs on every push / PR touching the SSOT, the vector corpus, or the gate itself.
+#
+# stdlib-only Python, no third-party deps, no network. The self-test runs FIRST and
+# must prove the gate is still falsifiable (every check FAIL-reachable) before the gate
+# is trusted against the live corpus.
+name: conformance-integrity-gate
+
+on:
+  push:
+    paths:
+      - "specs/numeric/formats_catalog.t27"
+      - "conformance/vectors/**"
+      - "tools/wp18_conformance_gate.py"
+      - "tools/wp18_selftest_gate.py"
+      - ".github/workflows/conformance-integrity-gate.yml"
+  pull_request:
+    paths:
+      - "specs/numeric/formats_catalog.t27"
+      - "conformance/vectors/**"
+      - "tools/wp18_conformance_gate.py"
+      - "tools/wp18_selftest_gate.py"
+      - ".github/workflows/conformance-integrity-gate.yml"
+  workflow_dispatch: {}
+
+permissions:
+  contents: read
+
+jobs:
+  integrity-gate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.11"
+
+      - name: Self-test (gate must be falsifiable)
+        # If any planted mutant is NOT killed, the gate is not trustworthy -> fail here.
+        working-directory: tools
+        run: python3 wp18_selftest_gate.py
+
+      - name: Run integrity gate against live corpus
+        run: |
+          python3 tools/wp18_conformance_gate.py \
+            --ssot specs/numeric/formats_catalog.t27 \
+            --vectors conformance/vectors \
+            --allowlist conformance/vectors/abs_error_allowlist.json \
+            --json wp18_report.json
+
+      - name: Upload integrity report
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: wp18-conformance-report
+          path: wp18_report.json
+          if-no-files-found: warn

conformance/integrity/fixtures/fixture_fp8_e5m2_posinf_roundtrip.json

@@ -0,0 +1,19 @@
+{
+  "fixture": "fp8_e5m2_pos_inf_special_roundtrip",
+  "kind": "special_value_row",
+  "expectation": "Check D2 MUST pass: +inf encodes and decodes to +inf; abs_error is a placeholder, not an arithmetic error; exempt from D and E.",
+  "pack_id": "fp8_e5m2",
+  "row": {
+    "name": "pos_inf",
+    "input_f64": "Inf",
+    "input_f64_hex": "0x7FF0000000000000",
+    "fp8_bits_hex": "0x7C",
+    "fp8_bits_int": 124,
+    "decoded_f64": "Inf",
+    "decoded_f64_hex": "0x7FF0000000000000",
+    "abs_error": 0.0,
+    "category": "inf"
+  },
+  "severity": "none",
+  "note": "Same-class round-trip inf->inf. The v1 gate wrongly flagged this as a nan-mismatch (gate handling gap); v2 D2 validates it correctly."
+}

conformance/integrity/fixtures/fixture_gf16_phi_mislabeled.json

@@ -0,0 +1,20 @@
+{
+  "fixture": "gf16_phi_mislabeled_abs_error",
+  "kind": "finite_value_row",
+  "expectation": "Check D MUST flag: stored abs_error 0.0 but abs(decoded-input) is nonzero.",
+  "pack_id": "gf16",
+  "row": {
+    "name": "phi",
+    "input_f64": 1.618033988749895,
+    "input_f64_hex": "0x3FF9E3779B97F4A8",
+    "gf16_bits_hex": "0x3F3C",
+    "gf16_bits_int": 16188,
+    "decoded_f64": 1.6171875,
+    "decoded_f64_hex": "0x3FF9E00000000000",
+    "abs_error": 0.0,
+    "category": "phi_anchor"
+  },
+  "rederived_abs_error": 0.0008464887498949025,
+  "severity": "Risk",
+  "note": "gf16 decode of phi rounds to 1.6171875; the 8.46e-04 error was stored as 0.0. Anchor identity phi^2+1/phi^2=3 still holds in f64; only the per-row abs_error label is wrong."
+}

conformance/vectors/INDEX_all_formats.json

@@ -321,7 +321,7 @@
       "file": "gf16_conformance_v0.json",
       "kind": "bitexact",
       "source": "hand-curated (pre-existing)",
-      "sha256": "7aea5b9e86ea71a54ae0c1601cea13e2d90d95fecaf2ae969eac1349cf7a2b42"
+      "sha256": "d1c0eb5bd66247b3c5db9a00a95e29cf4359653aec56f2f9e6827f96898d1509"
     },
     {
       "id": "gf20",

conformance/vectors/abs_error_allowlist.json

@@ -0,0 +1,54 @@
+{
+  "schema": "wp18-abs-error-allowlist/v1",
+  "note": "Explicit, auditable allow-list of bit-exact-pack rows that carry a NONZERO abs_error for an honest, documented reason. Every entry is a row where the round-trip is CORRECT but the input is not exactly representable (not_on_grid) or saturates (overflow_to_inf). The gate fails on any nonzero abs_error NOT listed here. Never add an entry to silence a real error; each must be a genuinely-disclosed case with a fixture.",
+  "allow": [
+    {
+      "pack_id": "bfloat16",
+      "row_name": "pos_0p1",
+      "reason": "not_on_grid",
+      "detail": "0.1 is not exactly representable in bf16; decoded_f64=0.10009765625, abs_error~9.77e-05 is the true nearest-representable error. Honest, expected."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "overflow_to_inf",
+      "reason": "overflow_to_inf",
+      "detail": "input 1e40 exceeds the gf16 dynamic range and saturates to +inf; abs_error=inf is the honest record of an out-of-range input, not an encode/decode bug."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "phi",
+      "reason": "not_on_grid",
+      "detail": "phi is not exactly representable on the gf16 grid; round-trip is correct, abs_error=0.000846489 is the true nearest-representable error (WP-18 F-1 correction)."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "inv_phi",
+      "reason": "not_on_grid",
+      "detail": "inv_phi is not exactly representable on the gf16 grid; round-trip is correct, abs_error=0.000130074 is the true nearest-representable error (WP-18 F-1 correction)."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "phi_squared",
+      "reason": "not_on_grid",
+      "detail": "phi_squared is not exactly representable on the gf16 grid; round-trip is correct, abs_error=0.000846489 is the true nearest-representable error (WP-18 F-1 correction)."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "inv_phi_sq",
+      "reason": "not_on_grid",
+      "detail": "inv_phi_sq is not exactly representable on the gf16 grid; round-trip is correct, abs_error=0.000130074 is the true nearest-representable error (WP-18 F-1 correction)."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "e",
+      "reason": "not_on_grid",
+      "detail": "e is not exactly representable on the gf16 grid; round-trip is correct, abs_error=0.000468172 is the true nearest-representable error (WP-18 F-1 correction)."
+    },
+    {
+      "pack_id": "gf16",
+      "row_name": "pi",
+      "reason": "not_on_grid",
+      "detail": "pi is not exactly representable on the gf16 grid; round-trip is correct, abs_error=0.000967654 is the true nearest-representable error (WP-18 F-1 correction)."
+    }
+  ]
+}

conformance/vectors/gf16_conformance_v0.json

@@ -87,7 +87,7 @@
       "gf16_bits_int": 16188,
       "decoded_f64": 1.6171875,
       "decoded_f64_hex": "0x3FF9E00000000000",
-      "abs_error": 0.0,
+      "abs_error": 0.0008464887498949025,
       "category": "phi_anchor"
     },
     {
@@ -98,7 +98,7 @@
       "gf16_bits_int": 15481,
       "decoded_f64": 0.6181640625,
       "decoded_f64_hex": "0x3FE3C80000000000",
-      "abs_error": 0.0,
+      "abs_error": 0.0001300737501052085,
       "category": "phi_anchor"
     },
     {
@@ -109,7 +109,7 @@
       "gf16_bits_int": 16542,
       "decoded_f64": 2.6171875,
       "decoded_f64_hex": "0x4004F00000000000",
-      "abs_error": 0.0,
+      "abs_error": 0.0008464887498949025,
       "category": "phi_anchor"
     },
     {
@@ -120,7 +120,7 @@
       "gf16_bits_int": 15118,
       "decoded_f64": 0.3818359375,
       "decoded_f64_hex": "0x3FD8700000000000",
-      "abs_error": 0.0,
+      "abs_error": 0.00013007375010509747,
       "category": "phi_anchor"
     },
     {
@@ -230,7 +230,7 @@
       "gf16_bits_int": 16568,
       "decoded_f64": 2.71875,
       "decoded_f64_hex": "0x4005C00000000000",
-      "abs_error": 0.0,
+      "abs_error": 0.0004681715409549092,
       "category": "normal"
     },
     {
@@ -241,7 +241,7 @@
       "gf16_bits_int": 16676,
       "decoded_f64": 3.140625,
       "decoded_f64_hex": "0x4009200000000000",
-      "abs_error": 0.0,
+      "abs_error": 0.000967653589793116,
       "category": "normal"
     }
   ]