fix(markdown): enforce the reference-label lookup limit

Port Bun’s MAX_LINK_LABEL_LEN lookup guard into Home. Definitions already reject labels over 999 bytes, but lookup still normalizes and searches oversized labels, violating the CommonMark limit and doing unnecessary allocation/work. Use one shared 999-byte constant for both definition parsing and lookup. Parent tracker: #66.

Pushed to origin/main in 2bd569b (fix(markdown): enforce reference label limits (#746)). The parser and lookup now share MAX_LINK_LABEL_LEN = 999, so overlong labels are rejected before normalization or map lookup while valid labels retain existing behavior. Verification completed: zig ast-check packages/runtime/src/md/ref_defs.zig; zig fmt --check packages/runtime/src/md/ref_defs.zig; git diff --check. Author and committer are glennmichael123 gtorregosa@gmail.com; no coauthors. Native semantic verification remains queued behind the serialized 3,840 MB build guard.