fix(js): close JS/WASM surface findings M2, M3, L1

[g4bri3lDev]

Summary

Closes three audit findings on the JavaScript/WASM surface (Task 4 of the audit-remediation plan):

• M2 — the mode matrix was fake. dithering.test.ts passed mode as ditherImage's third positional argument (DitherOptions, an object), so destructuring a Number yielded undefined for every field and all nine "produces valid output for mode %s" cases silently ran BURKES. Fixed the call to { mode }. Strengthened the per-mode test to diff each mode's output against DitherMode.NONE (so a broken kernel for that mode fails that mode's case) and added a cross-mode "not all identical" guard against future hollowing.
• M3 — wasm sentinel misfired. wasm/src/lib.rs's dither_image took scheme_id: u8; with non-empty palette_bytes, any parsable scheme_id silently enabled canonical pinning, so a direct wasm consumer leaving it at 0 got Mono-canonical pinning unasked. Changed the parameter to Option<u8>, made an unparsable Some(id) a hard error instead of a silent fallthrough, and dropped the 255 sentinel in core.ts in favor of undefined. This changes the wasm dither_image export's signature (not a breaking change for the TypeScript public API — ditherImage's TS surface is unchanged).
• L1 — unvalidated dimensions. core.ts returned the caller's claimed width/height while wasm derives height from buffer length, so a short buffer silently produced indices.length !== width * height. core.ts now validates data.length === width * height * 4 and throws, matching Python's existing cross-check style.

Test plan

• cd packages/javascript && bun run type-check — passes
• cd packages/javascript && bun run test — 44/44 pass
• cd packages/javascript && bun run build — passes
• cd packages/rust && cargo test --workspace — 61/61 pass (58 unit + 3 regression fixtures, untouched)
• cd packages/rust && cargo clippy --workspace -- -D warnings — clean
• python3 scripts/check_enum_parity.py — exits 0
• M2 proof: temporarily zeroed the FLOYD_STEINBERG kernel weights in algorithms.rs, rebuilt wasm — the produces valid output for mode 3 case (and the pre-existing serpentine test) failed; reverted, rebuilt, confirmed green and git status clean on that file.
• M3 proof: new test constructs a measured palette with an invalid scheme: 99 and asserts ditherImage throws instead of silently dithering without canonical pinning.
• L1 proof: new test passes an ImageBuffer whose data is shorter than width * height * 4 and asserts ditherImage throws.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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