fix(interp): eval_decl handles FnExtern (#328 build-failure root cause)

[hyperpolymath]

Summary

Unbreaks main's dune runtest gate — root cause of the 2-test failure baseline that's been red since #334 merged.

Root cause

eval_decl's TopFn fd arm matched fd.fd_body against only FnBlock and FnExpr, leaving FnExtern unmatched. Every prior interp test imported externs via use effects::{…} so this case never fired — the bug was latent.

The STDLIB-04a (#328) hermetic tests are the first to include an inline extern fn make_ref<T>(x: T) -> Ref<T> / Mut; declaration in the source they hand to Interp.eval_program. That triggers the missing arm and raises:

File "lib/interp.ml", line 1010, characters 16-21: Pattern matching failed
  Raised at Affinescript__Interp.eval_decl

Reproduces the 2 failures! in 0.088s. 298 tests run. line from every PR's build log since #334.

Fix

Wrap the body lowering in an outer match fd.fd_body with:

• FnExtern → Ok env (externs are runtime-bound via create_initial_env's VBuiltin table — panic / error / make_ref / get / set / etc., not via the AST).
• FnBlock _ | FnExpr _ → existing closure construction.

The inner inner-match's FnExtern arm is assert false with a comment — it's unreachable per the outer guard and the assert makes that explicit for the next reader.

Verification

The two failing tests (#328 make_ref/set/get round-trip (Int) and (String)) now have a defined evaluation path. The third 04a test (Deno codegen __cell shape) already passed — it bypasses interp.

Test plan

• CI build job: 2 failures! → 298 tests run clean
• CI lint: should also clear (was likely propagating from the same root)
• Hypatia DOC-FORMAT: no .md introduced

Refs #328.

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[github-actions]

🔍 Hypatia Security Scan

Findings: 133 issues detected

Severity	Count
🔴 Critical	15
🟠 High	62
🟡 Medium	56

⚠️ Action Required: Critical security issues found!

View findings

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[hyperpolymath]

Follow-up queued: broader "inline extern" fixture

This PR's fix is correct and scoped. Queueing a small follow-up for after this lands:

Add a test/e2e/fixtures/inline_extern_*.affine set covering the class of "inline extern fn declaration fed to every downstream consumer", not just the specific make_ref / error<T> paths that the STDLIB-04 tests now exercise.

The point of this PR is the missing FnExtern arm in Interp.eval_decl — survived since the interpreter was written because no test ever fed an inline extern fn to Interp.eval_program. STDLIB-04a was the first; PR #346 is the fix. To make sure the next latent gap of this shape (e.g. an extern with an effect row, a polymorphic extern with multiple type params, an extern type in a module that other modules import) surfaces against the gate rather than against the next agent, a small fixture set keyed on declaration shape — not on any one stdlib's externs — is worth ~30 lines of test code.

Suggested coverage:

• inline_extern_pure.affine — extern fn f(x: Int) -> Int; (no effects)
• inline_extern_effectful.affine — extern fn f(x: Int) -> Int / { IO };
• inline_extern_polymorphic.affine — extern fn id[T](x: T) -> T;
• inline_extern_type_consumed.affine — extern type Handle; extern fn use(h: Handle) -> Int;

Each fed through parse → resolve → typecheck → interp → at least one codegen target. The PR for this should Refs #346 and Refs the next CLAUDE.md "Test-fixture hygiene for latent bug surfaces" note (see claude/coordinator-feedback branch).

Not blocking on this PR; just don't want the fixture pattern lost in the merge.

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