Build wasm packs for every grammar

[zmaril]

Closes #146.

What changed

• use one shared grammar discovery command for native gates, WASM CI, default releases, and release rehearsal
• build and upload a WASM pack for every current grammar rather than a three-language allowlist
• parse each grammar's checked-in valid sweep fixture during direct pack checks
• rehearse positive and negative parsing through both Python and Node consumers for all nine fetched packs
• make build size reporting portable across GNU and BSD userlands

Verification

• TREEBANK_WASM_PYTHON=... ./tools/wasm-pack/check.sh — all 9 packs reproducible, loadable, provenance/roles exact, valid fixture clean
• TREEBANK_WASM_PYTHON=... ./tools/wasm-pack/test-release.sh — all 9 staged, fetched over HTTP, checksummed, parsed by both consumers, indexed, immutable on rerun
• cargo test --workspace
• actionlint .github/workflows/ci.yml
• bash -n tools/wasm-pack/*.sh
• shellcheck on changed shell entry points