feat(ctx-symbol): Anchor compact folds at AST body nodes and drive corpus regressions to zero

[Xuepoo]

Verdict: found already landed + one gap fixed

The M1 plan's core (Language::body_node_kinds(), find_body_node, fold_at_body_node) was already merged via earlier PRs (#13 et seq.) — not re-implemented. This delta re-measured the corpus, found and fixed the single remaining failure class, and closed out docs.

Baseline (branch @ 6cd7836)

files: 55498  raw-parse-ok slices: 1879020  regressions: 458
  css: 458

All h/hpp/go/py classes from the original ~5.3k baseline were already at zero.

Fix

• css rule sets: the brace-path fold emitted /* ... [N lines omitted] without the closing */; the unterminated comment swallowed the kept } and every folded rule set failed to re-parse. Fix appends Language::comment_close() in fold_at_body_node (no-op for line-comment backends). Commit: fix(ctx-symbol): fold css rule sets at their AST body (css).
• TDD: css_rule_set_fold_keeps_block_comment_closed.
• Also: two mechanical pre-existing clippy lint fixes in test files; added compact_output_is_byte_stable_across_invocations pinning byte-identical output across two invocations over all fold paths.
• Harness strictly decreased 458 → 0 in one iteration; final: files: 55498 raw-parse-ok slices: 1879020 regressions: 0.

Gates

cargo fmt --check / clippy --all-features --all-targets -D warnings / cargo test — all green (13 suites).

Report: https://github.com/Xuepoo/ctxctl-docs/pull/2