feat(review-tutor): analyze Dart and Rust, read neighbours, and ask from evidence

[ElbertePlinio]

Refs #62 — PR 4 of 4 (analyzer extensions and tutor integration). Stacked on #72; the final link of the batched Review Tutor chain (#67 → #69 → #71 → #72 → this). Closes the #62 PR plan once merged.

What changed

Analyzer (src/structure-languages.ts, src/structure-neighbours.ts, src/structure.ts)

• Dart (import / export / part / part of, package: mapping through a pubspec found by walking up from the importing file, dart: and other packages aggregated as external) and Rust (mod from crate roots and nested files, #[path] as an exact override read from masked source, use crate::… longest-prefix with brace-group expansion, pub use as re-export, super/self, include!) as private language tables with language-aware masking (nested comments, raw/triple strings, char literals).
• One-hop neighbours, default off: unchanged files that changed files import are read only through Git objects (git cat-file -s then blob; index for worktree/staged, the commit or range target otherwise; never for PR or pasted sources), outgoing only, with caps applied before work (50 files, 256 KiB each, 2 MiB total, 400 probes, 5 s with an abortable deadline) and every cap or refusal recorded as an explicit omission. Paths are validated before any Git call (.., absolute, -, :, control characters rejected).
• inputs.ts pins --src-prefix=a/ --dst-prefix=b/ for every local Git read, fixing wrong snapshot paths under diff.mnemonicPrefix; InputSnapshot gains revision / rangeTo.
• Protocol additions are additive: kinds part part-of mod use include, file status context, snapshot.neighbours, GET /api/structure?neighbours=1 (cache keyed by input + flag).

UI (src/page.ts, src/page-script.ts, src/page-styles.ts)

• One Include unchanged neighbours checkbox in the Structure header (always visible once a snapshot exists; unchecked and disabled with the reason in the partial note when unavailable; remembered per tab).
• Context files render muted (CONTEXT on the target where a neighbour appears, and as ordinary groups when they have their own rows); the new kinds render as text chips.
• Ask the tutor beside Open in Diff on evidence rows (list and graph): switches to Diff, selects the exact evidence line (the added line for modified edges) through the existing selection path, opens the inline composer with an empty question, and refuses while this tab's question is still running.

Tested

• 69 analyzer tests incl. all Dart/Rust fixtures, every neighbour cap and read path through a fake ExecFile, path rejection, determinism with reversed discovery order; real-Git end-to-end run against this repository. Page tests for the control's states, caching, persistence, focus restore, announcements, context rendering, the Ask flow on desktop and ≤860, modified-edge resolution, and the running-question guard. Rendered proof at 1440 / 390.
• bun run check: 552 passed | 25 skipped, 83.07% statements.

Not tested

• Real Dart/Rust repositories at scale (fixture-driven); package: resolution for packages outside the repo is by design external.

Local review: serious-backend panel + verification on the analyzer half; serious-user-facing panel + verification on the UI half. Human gate: merge only on Elberte's authorization.

[ElbertePlinio]

Ready for human review at 7869a39. Panels + verification clean; deferred low-severity items on #62.