watcher: changes OpenSave makes itself are not new saves

[ChakraFusion]

Part of #21.

The bug

Every change a sync applies looks exactly like the game saving. This has two effects:

• Snapshot floods. A peer deleting files one request at a time caused an auto-snapshot every couple of seconds. Each one held a save that was neither the old one nor the new one. They filled the retention budget and pushed out the snapshots that mattered.
• Echo. OnChanged fired for pulled files, so the sync was sent straight back out to the peers.

The fix

A new small package, internal/owntouch, records the paths OpenSave itself touches:

• pulled files (delta.PatchWriter);
• the folders a pull creates;
• a peer's deletions, and the files "keep theirs" removes;
• the mtimes "keep mine" re-dates.

The watcher classifies each burst of events against it:

• A burst made only of OpenSave's own changes moves the recorded hash and nothing else: no auto-snapshot and no OnChanged.
• A file counts as OpenSave's only if nothing wrote it after the mark (its mtime is not later). Once OpenSave is done with a file (renamed into place, re-dated to the peer's time), it records the exact time and size it left the file with. From then on the file is OpenSave's only while both still match. A game saving right after a sync is therefore still the game's change, even within the 2 seconds a plain time comparison would have to allow.
• A path found gone counts as OpenSave's only if OpenSave removed it (MarkRemoved). If the game or a person deletes a file OpenSave has just pulled, that deletion is theirs, and it is synced and snapshotted as such.
• The rescan after new folders come under watch counts as OpenSave's own only when OpenSave created those folders, for example a pull of a save with subfolders.

Tests

• owntouch_test.go:
  • a marked file and its folder count as ours, and a sibling doesn't;
  • a write after the mark is not ours;
  • a file deleted by someone else after OpenSave wrote it is not ours, while a file OpenSave removed is;
  • a same-size save written 20 ms after a finished pull is not ours.
• watcher/owntouch_test.go: own changes and own new folders are not snapshotted, and a burst mixing own changes with the game's is.
• go test ./... passes. CI on the fork (all jobs green): https://github.com/ChakraFusion/OpenSave/actions/runs/37174971888

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