feat(stdlib): close 6 STEP-3 Deno-ESM gaps surfaced by TS→AS ports (Refs #239, #242)

[hyperpolymath]

Closes the smallest, safest tractable gaps that the four merged STEP-2 TS→AffineScript ports — phronesis#19, session-sentinel#25, tropical-resource-typing#15, nafa-app#23 — flagged for STEP 3 of the estate-wide campaign (hyperpolymath/standards#239, sub-issue #242).

What lands

stdlib/Deno.affine (+6 externs)

extern	lowers to	gap
statIsFile(path) -> Bool	Deno.statSync(path).isFile	3
statIsDirectory(path) -> Bool	Deno.statSync(path).isDirectory	3
bytesLength(b) -> Int	(b).length	4
bytesByteAt(b, i) -> Int	(b)[i]	4
bytesAsciiSlice(b, s, e) -> String	String.fromCharCode(...(b).slice(s, e))	4
importMetaUrl() -> String	import.meta.url	5

The bytes* accessors give AffineScript first-class read access to the opaque Bytes returned by readFileBytes so callers can peek at file-magic / fixed-width binary headers without round-tripping through readTextFile (which throws on binary). importMetaUrl exposes the JS __dirname / __filename idiom for "find my own location"; only legal at module top level, which the Deno-ESM backend's output already is.

stdlib/string.affine (1 char)

fn ends_with → pub fn ends_with. The function existed but was private, so the four STEP-2 ports each inlined a string_sub-backed helper instead of importing it. Gap 2.

lib/codegen_deno.ml — wildcard let _ fix (gap 7)

StmtLet { sl_pat = PatWildcard _; … } now lowers to a bare expression statement instead of const _ = X;. Three back-to-back let _ = side() discards in the same scope tripped JS SyntaxError: Identifier '_' has already been declared. The wildcard pattern doesn't bind, so dropping the binding entirely preserves AffineScript semantics (evaluate for side effects).

// before
function discard_chain() {
  const _ = side(1);
  const _ = side(2);   // ❌ SyntaxError
  const _ = side(3);
  return 42;
}

// after
function discard_chain() {
  side(1);
  side(2);
  side(3);
  return 42;
}

Tests

tests/codegen-deno/deno_scripting_part2.{affine,deno.js,harness.mjs} — 11 assertions across the new lowerings + the wildcard fix. The harness reaching its discard-chain assertion is itself the test for gap 7: a syntax-error generated module would have thrown at the dynamic import line before any assert ran.

Verification

step	result
dune build bin/main.exe	✅
dune runtest	✅ 353/353
./tools/run_codegen_deno_tests.sh	✅ all harnesses (incl. the new one)

What does not land here (deferred deliberately)

• gap 1 — Deno.test extern lowering. The four merged STEP-2 ports use a panic-on-fail main() driver and it works; not unblocking STEP 4.
• gap 6 — native TOML parser. Heavier work; STEP-2 ports used regex field-presence checks where TOML mattered.

Gaps 9 (AFFINESCRIPT_STDLIB env discovery ladder, shipped #433) and 10 (negative integer literals, verified working in all contexts — match arms, array literals, equality, ternaries) were audited as already-resolved.

Refs

• Closes 6 of the 10 gaps blocking hyperpolymath/standards#242 (STEP 3 of #239).
• Unblocks STEP 4 (#243, mid-tier 4-9 file ports) and STEP 5 (#244, idaptik DLC) for any consumer that needs stat predicates, byte accessors, module-URL inspection, the public ends_with, or multiple let _ discards in a single scope.

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