feat(stdlib): STEP 4-A binary I/O — Bytes constructor + LE getters/setters (Refs #239, closes standards#326)

[hyperpolymath]

Adds the raw binary I/O surface that estate ABI-test ports need (raze-tui's 16-byte RazeEvent record, future tree-sitter-k9 / tree-sitter-a2ml grammar fixtures, half of bofig's contract tests).

Companion to the read-only bytesLength / bytesByteAt / bytesAsciiSlice accessors shipped in affinescript#504 (STEP 3 / standards#242). Together the two PRs give AffineScript first-class binary-buffer support.

What lands

stdlib/Deno.affine (+10 externs)

extern	lowers to	notes
bytes_new(n) -> Bytes	new Uint8Array(n)	zeroed
bytes_fill(n, byte) -> Bytes	(new Uint8Array(n)).fill(byte & 0xFF)	all-byte buffer
bytes_set_u8(b, off, v) -> Int	… .setUint8(off, v & 0xFF), 0	returns 0
bytes_set_u16_le(b, off, v) -> Int	… .setUint16(off, v & 0xFFFF, true), 0	LE
bytes_set_u32_le(b, off, v) -> Int	… .setUint32(off, v >>> 0, true), 0	LE
bytes_set_i32_le(b, off, v) -> Int	… .setInt32(off, v | 0, true), 0	LE
bytes_get_u8(b, off) -> Int	… .getUint8(off)
bytes_get_u16_le(b, off) -> Int	… .getUint16(off, true)	LE
bytes_get_u32_le(b, off) -> Int	… .getUint32(off, true)	LE
bytes_get_i32_le(b, off) -> Int	… .getInt32(off, true)	LE

All multi-byte integer variants are little-endian — the estate's C ABI contracts (raze-tui raze-events.ads, Idris2 Events.idr) are LE-pinned. Setters return Int = 0 so they compose in an expression-statement position; the caller is responsible for the buffer-bounds invariant (an out-of-range offset throws RangeError at the host boundary). Bounds-check via bytesLength from #504.

Tests

tests/codegen-deno/bytes_binary_io.{affine,deno.js,harness.mjs} — round-trips a raze-tui-shaped RazeEvent record (LE i32 + u32 + u8 + u16 × 2) with field-level equality, plus boundary cases:

• u32 max (0xFFFFFFFF) round-trips
• i32 -1 preserves sign
• LE byte order verified byte-by-byte via DataView (0x12345678 writes low byte at offset 4, high byte at offset 7)
• bytes_fill masks 256 → 0, -1 → 0xFF, 0xFF → 0xFF
• bytes_new produces a zero-initialised buffer

Verification

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

Out of scope

• Big-endian variants — not needed by any current estate ABI; revisit if/when an external API forces BE.
• 64-bit getters/setters (*_i64_le) — defer; current STEP 4 candidates max out at 32-bit.

Relation to #504 (STEP 3)

Both this PR and #504 add externs to stdlib/Deno.affine and lowerings to lib/codegen_deno.ml. The two sets are disjoint at the symbol level:

• feat(stdlib): close 6 STEP-3 Deno-ESM gaps surfaced by TS→AS ports (Refs #239, #242) #504: statIsFile, statIsDirectory, bytesLength, bytesByteAt, bytesAsciiSlice, importMetaUrl, pub fn ends_with, wildcard-let codegen fix.
• this PR: 10 binary I/O externs in a separate section.

The bytes test fixture uses uniquely-named let _r0 = …, let _r4 = …, etc. instead of the let _ = … form so this PR is independently mergeable from #504; once #504 lands, the fixture could be simplified to use the wildcard pattern.

Refs

• Closes standards#326
• Refs: standards#239 (umbrella), standards#242 / feat(stdlib): close 6 STEP-3 Deno-ESM gaps surfaced by TS→AS ports (Refs #239, #242) #504 (STEP 3 predecessor), standards#243 (STEP 4 per-repo unblock target)

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