- **Cross-cutting bug sweep (10+ root-cause fixes).**
  - *Compiler:* a function or program body whose bytecode exceeded 64 KiB
    silently truncated its `u16` jump operands and jumped somewhere wild at
    runtime; every finalized chunk is now guarded and oversized bodies fail
    compilation with a clear "split it into smaller functions" error.
  - *Typechecker:* optional subscript access on a shape (`x?["k"]`) dropped
    the optional flag, so an unknown key inferred gradual instead of `nil`
    and mismatched assignments passed silently.
  - *Stdlib:* the regex builtins (`regex_match`, `regex_replace`,
    `regex_captures`, `regex_split`) stringified an explicit `nil`
    pattern/text/replacement to the literal string `"nil"` and compiled —
    and matched — it as a real regex; `nil` now takes the same fallback as a
    missing argument (mirroring the existing flags guard).
  - *Formatter:* `harn fmt` relocated comments to the end of the file from
    three positions — trailing comments on inline match arms
    (`1 -> { x } // c`), trailing comments on dict/struct-literal entries,
    and full-line comments inside multiline dict/list literals. All three now
    stay anchored, and a literal with interior comments keeps its multiline
    shape.
  - *Linter:* `comparison-to-bool` flagged optional-chained presence tests
    (`d?.enabled == false`) as "redundant" and its autofix rewrote them to
    `!d?.enabled`, which flips the branch when the chain is nil; the rule now
    skips operands that can be nil via optional chaining. Also,
    `prefer-optional-shorthand` diagnostics reported byte-based columns,
    overshooting on lines with multibyte characters; columns are now
    character-based.
  - *Spec:* `spec/HARN_SPEC.md` claimed `if`/`else` and `match` execute
    without creating a child scope, contradicting both the docs and actual
    behavior (branch/arm bodies are block-scoped); the scope-creation section
    now matches the implementation.
  - *MCP test fixtures:* `FakeServerBehavior::HeaderMismatch` was declared
    but never implemented; the fake RC server now validates `Mcp-Method` /
    `Mcp-Name` headers via the production negotiation helper and answers
    `-32600` with the spec error shape.
  - *Docs/release:* `docs/src/embedding-rust.md` pinned `tag = "v0.8.57"`
    (~46 releases stale); the pins are refreshed and
    `release_gate.sh prepare` now bumps them automatically each release.
