feat: add JSON as a second canonicalized language

[bdelanghe]

Summary

Adds JSON as a second canonicalized language in git-ast's clean/smudge round-trip, alongside the Rust support from #26. JSON is additive: it slots into the per-extension dispatch in filters::transform — no change to the pkt-line filter-process protocol, the printer, or pktline.

Rebased onto current main (#26). An earlier revision of this branch was built on a stale pre-#26 base; it has been replaced with this additive integration — no conflicts, no duplication.

• src/json.rs — canonicalize() parses with serde_json and re-emits a deterministic canonical form (object keys sorted, pretty-printed, trailing newline). Same contract as printer::canonicalize; fail-closed on invalid JSON (rejects the commit).
• src/filters.rs — transform() now dispatches by extension: .rs → printer, .json → json, smudge identity, pass-through otherwise.
• src/setup.rs — routes both *.rs and *.json in .gitattributes (idempotent).
• src/main.rs / src/lib.rs — help text + crate docs updated for two languages.

Design note

Canonical form is sorted-key pretty JSON, not compact RFC-8785 JCS — same value-level normalization (sorted keys, deterministic scalars), but one value per line so the filter's cleaner diffs purpose holds. serde_json only (its Map is a BTreeMap, so keys sort and scalars format deterministically).

Tests — all green (31 unit + 10 scenarios / 40 steps)

• src/json.rs (6): sort/normalize, idempotence, key-order independence, value fidelity, trailing newline, reject-invalid.
• src/filters.rs (2): clean canonicalizes JSON over the protocol; invalid JSON yields status=error.
• cucumber claims (tests/features/claims.feature, 4 new): reformatting → no diff, byte-identical blobs, checkout round-trip, fail-closed — driving real git. The install step now routes *.json too.
• cargo fmt --all -- --check + cargo clippy --all-targets -- -D warnings clean.

PR run sheet

1. Independent PR — only adds the JSON language; no bundled/speculative changes
2. Changed codepaths verified — unit + protocol + 4 real-git cucumber scenarios
3. Root cause identified — n/a (additive feature)
4. No duplication — reuses feat: working clean/smudge round-trip for a Rust subset #26's transform/pktline/setup; one dispatch arm + one language module
5. No unrelated changes — README/help/docs scoped to the new language

Trust ledger

Moves row 8.1 toward 🟡: a second language now rides the same working pipeline. Rust remains a documented subset; structural diff/merge (node identity) stays out of scope.

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