[ilyaZar]

Follow-up to #138.

#138 moved directory rescans from `localIndex` into goroutines, because `/rest/db/scan` answers only when the scan ends, which waits for any running pull. Nothing limited them: each `LocalIndexUpdated` event could start up to 16, each waiting up to the 8 s response timeout, and repeated events for the same directories during a pull piled them up.

Changes:

• `rescanSubdirectory` keeps the directories whose rescan is still running and does not ask for them again; once a rescan returns, the directory can be rescanned again
• bundled binaries and checksums rebuilt

Testing

• new `TestSubdirectoryRescanIsNotRepeatedWhileInFlight`: a second request for a directory whose scan is blocked in Syncthing is skipped, and a later one goes through; it fails without the guard
• `go -C core test -race ./...` and `go -C core vet ./...` pass
• `packaging/bundled/build.sh` with go1.23.12; `packaging/bundled/verify.sh` passes
• `tests/native-core-architecture.test.sh` passes