* filer.conf: merge fsync on presence like worm
fsync is a pure preference, so OR-merging it meant a nested rule could
never turn a parent's fsync off. Make PathConf.fsync optional and merge
it most-specific-wins, matching worm: an unset rule inherits, an explicit
value overrides in either direction. readOnly and disableChunkDeletion
stay OR'ed — they are fail-closed locks quota enforcement depends on.
Pre-version-2 configs were written with EmitUnpopulated, so every rule
carries an explicit fsync=false that meant nothing; it is dropped to
unset on load so it does not read as an opt-out.
Generated with [Devin](https://devin.ai)
Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
* filer.conf: test fsync override survives a save/reload round trip
---------
Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>
