test(core): gate v0.4 format chunk recovery

[hunhee98]

Summary

• add a v0.4 format chunk recovery fixture suite for Java, TSX, HTML, CSS/SCSS, Markdown/MDX, YAML/JSON/TOML, Dockerfile, and Shell
• add a deterministic format_recovery bench that reports format_chunk_recovery_pct
• wire format_chunk_recovery_pct into scripts/regression-gate.py and benchmarks/baseline.json at a strict 100% functional gate
• update README, ROADMAP, CHANGELOG, and benchmark docs

Release lane

• minor-train
• target: v0.4.0

Test plan

• cargo bench -p pluck-core --bench format_recovery
• cargo fmt --all -- --check
• python3 scripts/version-check.py --allow-unreleased
• git diff --check
• cargo test --all -- --skip watcher
• env RUSTFLAGS=-D warnings cargo check -p pluck-core --benches

Notes

• Local full scripts/regression-gate.py parsed the new metric at 100%, but failed on the existing freshness_p50_ms_medium metric on macOS with a 5011ms measurement. PR CI should be treated as the Ubuntu gate result.