T6 of the external code review. Adds docs/PARSER_LIMITATIONS.md: which
constructs the reverse parser recognises, which it deliberately
simplifies (try/catch, generators, await, nested classes, multiple
functions - only the first becomes a diagram), and what happens when a
construct is outside the subset (warning plus a plain action node). A
separate section records why the parser is regex-based rather than
typescript-estree: AGENTS.md forbids runtime dependencies and the v1
subset does not justify bundling a full TypeScript parser.

Links the new document from the AGENTS.md map, CODEGEN.md and ROADMAP.md,
closes the T3 (warnings reach the UI) and T4 (property tests) roadmap
items, and moves the real-TS-parser switch into 'deliberately deferred'.
CHANGELOG gains an 'external code review' section covering T1-T5, with the
rejected findings recorded alongside the accepted ones.