Extend C-210 to every bytes float writer

[O6lvl4]

Implements the owner's ruling on almide/almide#3658 (2026-10-13): C-210 extends to every bytes float writer.

Contract

C-210 named only the appends (append_f64_le/be, append_f32_le/be). Its statement now covers the whole family under one rule: the appends, the in-place sets (set_f64_le/be, set_f32_le/be, set_float32) and the cursor writes (write_f64_be, write_float32). Each writes the canonical NaN on every leg: f64 0x7FF8000000000000, f32 0x7FC00000. The rest of the statement is unchanged.

Fixtures

• New: spec/wasm_cross/nan_canonical_bytes_set_write.almd. It sends five values through every set and write writer: 0.0/0.0, a from_bits payload NaN (sign set, low payload), a negated NaN, an arithmetic NaN, and 1.5. The 1.5 row shows that non-NaN bits stay raw.
• Updated: spec/wasm_cross/nan_canonical_bytes_write.almd now matches the implementation's copy. The implementation added the #3647 rows (payload, negated and arithmetic NaN through all four appends) and they had not reached the judge.

Reference evaluator

ref/ wrote raw bits on every float writer, the appends included, so on a payload NaN it disagreed with C-210. Now:

• all of its float writers (write_bytes_fn for append/set, write_float32, write_f32_le, write_f64_be) take their bits from one helper pair, obs_bits_f64 / obs_bits_f32;
• it implements bytes.set_float32;
• write_float32 accepts a Float32 value. It used to abstain with a type mismatch.

On the reference, both NaN fixtures now print the canonical bytes.

Gates (run locally)

All green:

• check-contracts, check-contract-provenance, check-als-element-coverage, check-als-style, check-als-validation, check-links, check-gate-verification, selftest-conformance, check-runner-coverage, check-ratchet-separation --staged
• check-ref-kernel, check-ref-totality: 733 evaluated, 18 abstained, ceiling 18
• cargo fmt --check and cargo clippy -D warnings on ref/

Regenerated: docs/contracts/README.md, docs/contracts/conformance.md.

After this merges, the implementation advances proofs/als-pin.txt, then lands native and wasm canonicalization on its branch fix-3658-float-writer-nan.