Stop the file watcher from silently truncating its change list

[Seungpyo1007]

The bug

The file watcher collected changed paths with one line:

if (touched.size < MAX_TOUCHED) touched.add(...)   // MAX_TOUCHED = 2000

Past the cap it dropped paths silently. The app itself is fine — syncFromDisk re-reads the whole tree anyway. The victim is the extension file watcher: a dropped path becomes "nothing happened" for that extension, which then believes its index is complete.

The fix

electron/touchSet.cjs owns the bag now. The cap is 20000, and when it is exceeded the notification carries overflow: true. On overflow the renderer stops narrowing by the reported names and diffs the full before/after tree instead, so a dropped name cannot silently vanish. Only when the tree is also truncated do we genuinely not know, and that case is logged rather than passed off as complete.

What the probe disproved

This was built on the assumption that a branch switch overruns the 2000 cap. It does not. Creating 2500 files at once in the real app produced watcher notifications carrying ~180 names, not 2500 — fs.watch itself drops events long before our cap is reached (the Windows recursive watcher's kernel buffer overflows).

So the honest scope of this change is narrower than the framing suggests: it guarantees the app never truncates the list itself. Whether every name arrives is decided by the OS.

The app already survives that. syncFromDisk re-reads the tree and classify recovers created/deleted from the full diff — the probe confirms all 2500 files land in the tree. What is lost is changed events, and closing that properly needs content hashing or a polling fallback. Left as separate work.

Verification

• 9 unit tests: overflow reporting, dropped count, duplicates not consuming the cap, backslash normalization, flag cleared on drain. 981 passing (was 972).
• Real-app probe, 3/3: nothing truncated on our side (overflow=false), every reported path is a real path, all 2500 files present in the tree.
• npm run typecheck clean. Line endings unchanged — main.cjs 1905 CRLF / 0 LF, preload.cjs 253 LF / 0 CRLF.