fix(watch): detect_changes returns every source file each cycle; --filter only gates the trigger #578

Problem. commands/watch.rs::detect_changes walks the project and returns every source file that passes the extension/git-only/glob filters on each debounce tick, rather than files that actually changed. After a trigger the whole project is incrementally indexed, so --filter only decides whether a cycle fires; it does not restrict which files get indexed.

Repro (verified on develop @ 1dd7ac4). Project with src/a.rs and src/b.rs; run cora watch --filter 'a.rs'; append a line to src/b.rs only. Output: Reindexed: 1 files, and the stored fingerprint for b.rs changes even though it does not match the filter.

Why it matters now. Before #577 a basename filter like a.rs never matched (anchored glob), so nothing triggered. Since #577 basename patterns match, so every 500 ms cycle triggers a full incremental index run. It is cheap per cycle (unchanged files are skipped by fingerprint) but is a poll over the whole tree. index --watch also uses this path since #576.

Direction. detect_changes should compare against the last-seen mtime/size (or use fs events) and return only changed files; --filter should limit which files are indexed, not just which cycles fire. Add tests that a change to a non-matching file produces no reindex.

Found while smoke-testing #577. Related: #576.