fix: codec_sweep.sh stub honesty check must FAIL THE SCRIPT, not just log

Codex P2 review catch — the "Stub honesty check" block was echoing
results[0].stub but always exiting 0, which undermined the anti-#219
safeguard the script claims to enforce. In Phase 0/2 runs, a non-stub
or malformed response could look like a successful sweep in automation.

Fix: after extracting the flag, case on it:
  - true  → OK, echo success (Phase 0 stub honored)
  - false → FAIL exit 3, diagnostic message naming two possible causes
            (server running non-scaffold code OR wrong endpoint hit)
  - *     → FAIL exit 3, points at the malformed response section

The fix embodies the session's own principle ("the object does the
work" / "stub flag is machine-checkable anti-#219"): a script that
claims to check a flag but doesn't exit on failure is exactly the
pattern we warn against in CODING_PRACTICES.md. The script now
actually enforces what it documents.

Cross-ref: PR #239 D3.2; EPIPHANIES.md 2026-04-20 "D0.2 stub flag is
anti-#219 defense at the type level"; Codex P2 review 2026-04-21.

https://claude.ai/code/session_01SbYsmmbPf9YQuYbHZN52Zh

Author: claude

scripts/codec_sweep.sh

@@ -56,6 +56,33 @@ echo "$response" | jq '.'
 
 echo
 echo "=== Stub honesty check ==="
-stub_flag=$(echo "$response" | jq '.results[0].stub // "no results"')
+# Per EPIPHANIES.md 2026-04-20 "D0.2 stub flag is anti-#219 defense at
+# the type level" — the check MUST fail the script (not just log) when
+# the flag is absent or false. Until D2.2 lands real decode-and-compare,
+# Phase 0/2 runs return stub:true. A non-stub response here means
+# either the wrong endpoint was hit, the response was malformed, or
+# (worst case) the server silently shipped non-stub code and this
+# script is now pretending synthetic numbers are real.
+
+stub_flag=$(echo "$response" | jq -r '.results[0].stub // "missing"')
 echo "results[0].stub = $stub_flag"
-echo "Expected: true (Phase 0 stub; D2.2 flips to false when real decode lands)."
+
+case "$stub_flag" in
+    true)
+        echo "OK — Phase 0 stub honored. (D2.2 will flip this to false when real decode lands;"
+        echo "     at that point, flip this check too.)"
+        ;;
+    false)
+        echo "FAIL — results[0].stub is false but D2.2 has not landed." >&2
+        echo "        This script refuses to treat non-stub output as real during Phase 0." >&2
+        echo "        Either the server is running non-scaffold code (update this check)," >&2
+        echo "        or the request hit the wrong endpoint / unexpected handler." >&2
+        exit 3
+        ;;
+    *)
+        echo "FAIL — results[0].stub missing or unparseable (got: $stub_flag)." >&2
+        echo "        Response may be malformed or an error payload." >&2
+        echo "        Inspect the --- response --- section above." >&2
+        exit 3
+        ;;
+esac