fix(interp): wire missing string_length builtin (Refs #332, #329)

[hyperpolymath]

Summary

string_length : String -> Int / Pure is declared as a primitive in three places that AffineScript programs traverse during the compile pipeline:

Layer	File	State
Surface decl	stdlib/effects.affine	✓ declared
Resolver	lib/resolve.ml	✓ recognised
Typechecker	lib/typecheck.ml	✓ typed String -> Int / EPure
Codegen (Deno)	lib/codegen_deno.ml	✓ (s).length
Interpreter	lib/interp.ml	❌ MISSING VBuiltin entry

Programs calling string_length compile cleanly and fail at run with Unbound variable: string_length.

What this breaks (without the fix)

Two E2E test suites currently red on main:

• E2E STDLIB-04e Pure #332 / "string_length(\"hello\") == 5" — direct miss.
• E2E STDLIB-04b error #329 / error_diverges_string_call_site — uses string_length(k) > 0 as a guard, so the missing binding masked the RuntimeError "empty key" assertion as an Unbound variable error.

Fix

One-block addition to create_initial_env's builtin table, right after int_to_string / float_to_string. Matches the resolver/typechecker contract and the codegen-Deno semantics. 14 lines including the explanatory comment.

("string_length", VBuiltin ("string_length", fun args ->
  match args with
  | [VString s] -> Ok (VInt (String.length s))
  | _ -> Error (TypeMismatch "string_length expects String")
));

How this was found

affinescript#361's build job was red after that PR's bench/dune + fixture-exemption fixes cleared the other two baselines. The two failing E2E tests both pointed at the same root cause — surface-decl/resolver/typechecker all knew string_length, but the interp didn't.

Test plan

• dune runtest passes the STDLIB-04e suite (4 cases) and STDLIB-04b suite
• No regression in Lexer / Parser / Typechecker / Resolver / other E2E suites (this is purely additive in the builtin table)

Refs #332 (STDLIB-04e), Refs #329 (STDLIB-04b). Companion to #361 — landing this first will turn #361's build job green.

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 112 issues detected

Severity	Count
🔴 Critical	15
🟠 High	47
🟡 Medium	50

⚠️ Action Required: Critical security issues found!

View findings

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