feat(stdlib): STDLIB-04a — Mut effect externs (make_ref/get/set) → real impl (Closes #328)

[hyperpolymath]

Summary

Lands real implementations of the three Mut effect externs declared in stdlib/effects.affine:

extern fn make_ref<T>(x: T) -> Ref<T> / Mut;
extern fn get<T>(r: Ref<T>) -> T / Mut;
extern fn set<T>(r: Ref<T>, x: T) -> Unit / Mut;

Before this PR they were stubs — the surface parsed and typechecked, but no backend wired the runtime semantics, so any caller would compile and then fail at runtime with make_ref is not defined.

Changes

• lib/interp.ml — three new VBuiltin entries: make_ref allocates a VMut cell (the existing runtime mutable-cell variant in Value); get/set route through the standard deref / assign primitives. Reuses the same store the borrow-surface &mut already uses.
• lib/codegen_deno.ml — three new entries in deno_builtins: make_ref ��� {__cell: x}, get → ((r).__cell), set → (((r).__cell = x), null). Single-field object so get/set are O(1); set returns null (Unit) via comma-expression to match the signature.
• test/test_e2e.ml — three hermetic tests under E2E STDLIB-04a Mut #328:
  1. Int round-trip: make_ref(7); set(r, 42); get(r) == 42
  2. String round-trip (value-polymorphic): make_ref("alpha"); set(r, "omega"); get(r) == "omega"
  3. Deno codegen emits __cell shape (proves the new table entries actually fire).
• docs/TECH-DEBT.adoc — row 04a marked DONE per the audit-split contract from docs(tech-debt): split STDLIB-04 into 04a–04e per per-extern audit (Refs #175) #333.

Conceptual note

These are runtime mutable cells (Ref<T> parameterised type), distinct from the borrow-checker's &/&mut references — different concept that happens to share the word "ref". The Mut effect on the signatures is what marks the call sites as observably stateful.

Test plan

• Three new hermetic tests added to the gate (E2E STDLIB-04a Mut #328)
• No change to stdlib AOT gate count (no new stdlib file)
• CI: dune runtest green (gate moves from 281 → 284)
• Hypatia DOC-FORMAT: no .md introduced

Closes #328. Refs #175.

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🔍 Hypatia Security Scan

Findings: 143 issues detected

Severity	Count
🔴 Critical	13
🟠 High	70
🟡 Medium	60

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