Add cross-impl coercion conformance + unify roundtrip harnesses

[namingbe]

The roundtrip suite only ever sees valid input, so coercion (decoding raw byte sequences) was never cross-checked: champernowne-fixed is a Python-built, already-valid fixed point, and roundtripping it exercises zero coercion.

Add scripts/coercion.sh + per-impl harnesses (.github/coercion/): each implementation coerces the enum corpus and raw champernowne.nsv via encode(decode(x)) and emits a sha256 manifest; the coercion-compare CI job diffs the manifests across Python/JS/Java/Scala/Rust — the cross-impl consensus is the oracle, no single implementation is trusted. Each run also asserts its output is a fixed point and that champernowne's coercion reproduces the committed champernowne-fixed.nsv byte for byte.

A second, stream mode feeds the enum corpus (which includes unterminated inputs the roundtrip suite never sees) through each implementation's resumable Reader + Writer; coercion-compare-stream is expected red until reader EOF semantics are aligned (Python/JS emit the incomplete trailing row where Java/Rust/Scala buffer it, and the JS streaming Reader corrupts empty rows) — it exists to keep that divergence visible.

Also rewrite Roundtrip.scala to use batch encode/decode like every other roundtrip harness (streaming coverage now lives in the coercion stream mode). Its champernowne-size batch decodes need -Xmx12g, since nsv-scala's decode materializes per-char tuples.

Verified locally: batch manifests are byte-identical across Python/JS/Java/Scala/Rust on enum + full raw champernowne, every impl reproduces champernowne-fixed.nsv, and the documented stream split reproduces (Java=Rust=Scala; Python diverges on 970 unterminated inputs; JS on 1038).

https://claude.ai/code/session_012yq2rjyRY1SRcsY5hnsjzk