ci: add ABI↔FFI conformance gate (#34)

* Auto-install pinned Zig toolchain in setup + devcontainer

The Zig FFI bridge is half of the ABI-FFI standard, but nothing installed
Zig: .tool-versions only lists it (commented), setup.sh stops at `just`,
and the devcontainer's `postCreateCommand: just deps` referenced a `deps`
recipe that did not exist. Unlike the other toolchain pieces, Zig is not
distributed via GitHub releases, so it must come from ziglang.org.

Add scripts/install-zig.sh: an idempotent, fail-soft installer for the
pinned Zig 0.14.0 (arch/OS-aware, uses the system CA store the agent proxy
populates, never --insecure). If ziglang.org is not on the session's egress
allowlist the download 403s and the script exits 0 with an actionable
message, so it never blocks setup or a session.

Wire it in via the two paths the project already uses: a "Step 1b" in
setup.sh (where the template exposes that step), and a new `deps` Justfile
recipe backing the devcontainer postCreateCommand. Once ziglang.org is
allowlisted, future setups and dev containers install Zig automatically.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH

* ci: add ABI↔FFI conformance gate

Adds scripts/abi-ffi-gate.py (fails on a declared ABI C function with no Zig
export, a mismatched result-code map, or an unrendered template token) and a
.github/workflows/abi-ffi-gate.yml that runs it plus a Zig 0.14.0 build/test
of the FFI. The Idris2 ABI is the source of truth.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/abi-ffi-gate.yml

@@ -0,0 +1,38 @@
+# SPDX-License-Identifier: MPL-2.0
+# abi-ffi-gate.yml — enforce that the Zig FFI conforms to the Idris2 ABI.
+#
+# The Idris2 ABI (src/interface/abi) is the source of truth. This gate fails if
+# the Zig FFI (src/interface/ffi) drifts from it: a declared C function with no
+# export, a mismatched result-code map, or an unrendered template token. A
+# second job builds + tests the Zig FFI under the pinned Zig 0.14.0.
+name: ABI-FFI Gate
+
+on:
+  pull_request:
+  push:
+    branches: [main, master]
+
+permissions:
+  contents: read
+
+jobs:
+  conformance:
+    name: ABI ↔ FFI structural conformance
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run ABI-FFI gate
+        run: python3 scripts/abi-ffi-gate.py
+
+  zig-build:
+    name: Zig FFI builds + tests (Zig 0.14.0)
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Zig 0.14.0
+        run: |
+          curl -fsSL https://ziglang.org/download/0.14.0/zig-linux-x86_64-0.14.0.tar.xz -o /tmp/zig.tar.xz
+          tar -xf /tmp/zig.tar.xz -C /tmp
+          echo "/tmp/zig-linux-x86_64-0.14.0" >> "$GITHUB_PATH"
+      - name: zig test FFI
+        run: zig test src/interface/ffi/src/main.zig -lc

scripts/abi-ffi-gate.py

@@ -0,0 +1,103 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: MPL-2.0
+# Copyright (c) 2026 Jonathan D.A. Jewell (hyperpolymath) <j.d.a.jewell@open.ac.uk>
+#
+# abi-ffi-gate.py — fail (exit 1) if the Zig FFI does not conform to the Idris2
+# ABI. The Idris2 ABI is the source of truth. Checks, with no toolchain needed:
+#
+#   1. the Zig FFI carries no unrendered `{{...}}` template tokens;
+#   2. every `%foreign "C:<name>"` symbol declared anywhere in the ABI .idr
+#      sources is exported by the Zig FFI (`export fn <name>`);
+#   3. the Zig `Result = enum(c_int)` and the Idris `resultToInt` agree on BOTH
+#      names and integer values (the `Error`/`err` spelling is treated as one).
+#
+# Usage: python3 scripts/abi-ffi-gate.py [repo_root]   (defaults to cwd)
+
+import os
+import re
+import sys
+import glob
+
+
+def camel_to_snake(s):
+    return re.sub(r"(?<!^)(?=[A-Z])", "_", s).lower()
+
+
+def canon_rc(name):
+    n = name.lower()
+    return "error" if n in ("err", "error") else n
+
+
+def find_result_enum(zig):
+    """Return {variant: value} for the C-ABI Result enum, or {}."""
+    best = {}
+    for m in re.finditer(r"enum\s*\(\s*c_int\s*\)\s*\{(.*?)\}", zig, re.S):
+        body = m.group(1)
+        variants = {}
+        for vm in re.finditer(r'@?"?([A-Za-z_][A-Za-z0-9_]*)"?\s*=\s*(\d+)', body):
+            variants[canon_rc(vm.group(1))] = int(vm.group(2))
+        # The Result enum is the one starting at ok = 0.
+        if variants.get("ok") == 0 and len(variants) > len(best):
+            best = variants
+    return best
+
+
+def main():
+    root = sys.argv[1] if len(sys.argv) > 1 else "."
+    name = os.path.basename(os.path.abspath(root))
+    abi_dir = os.path.join(root, "src/interface/abi")
+    zig_path = os.path.join(root, "src/interface/ffi/src/main.zig")
+    errs = []
+
+    idr_files = [
+        p for p in glob.glob(os.path.join(abi_dir, "**", "*.idr"), recursive=True)
+        if os.sep + "build" + os.sep not in p
+    ]
+    if not idr_files:
+        print(f"ABI-FFI GATE: SKIP ({name}) — no Idris2 ABI .idr files under {abi_dir}")
+        return 0
+    if not os.path.exists(zig_path):
+        print(f"ABI-FFI GATE: FAIL ({name}) — no Zig FFI at {zig_path}")
+        return 1
+
+    idr = "\n".join(open(p, encoding="utf-8").read() for p in idr_files)
+    zig = open(zig_path, encoding="utf-8").read()
+
+    # 1. unrendered template tokens
+    toks = sorted(set(re.findall(r"\{\{[A-Za-z0-9_]+\}\}", zig)))
+    if toks:
+        errs.append(f"Zig FFI has unrendered template tokens: {toks}")
+
+    # 2. foreign C symbols must be exported
+    csyms = sorted(set(re.findall(r"C:([A-Za-z0-9_]+)", idr)))
+    exports = set(re.findall(r"export fn ([A-Za-z0-9_]+)", zig))
+    missing = [s for s in csyms if s not in exports]
+    if missing:
+        errs.append(f"{len(missing)} ABI function(s) not exported by the Zig FFI: {missing}")
+
+    # 3. result-code map (names + values) must agree
+    idr_rc = {}
+    for m in re.finditer(r"resultToInt\s+([A-Za-z0-9]+)\s*=\s*(\d+)", idr):
+        idr_rc[canon_rc(camel_to_snake(m.group(1)))] = int(m.group(2))
+    zig_rc = find_result_enum(zig)
+    if idr_rc and not zig_rc:
+        errs.append("no Zig `enum(c_int)` Result block (with `ok = 0`) found to compare result codes")
+    elif idr_rc and zig_rc and idr_rc != zig_rc:
+        errs.append(
+            "Result-code map differs (name or value):\n"
+            f"      Idris resultToInt: {dict(sorted(idr_rc.items()))}\n"
+            f"      Zig   Result enum: {dict(sorted(zig_rc.items()))}"
+        )
+
+    if errs:
+        print(f"ABI-FFI GATE: FAIL ({name})")
+        for e in errs:
+            print("  - " + e)
+        return 1
+    print(f"ABI-FFI GATE: OK ({name}) — {len(csyms)} ABI functions exported, "
+          f"{len(idr_rc)} result codes match")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())