fix some build warnings

[Anarchid]

  RECORD_HEADER_SIZE — the spookiest one. Introduced in 3b5b891 (Initial release, co-authored by Claude Opus 4.5). The math is correct: 4+1+1+8+8+8+8 = 38 bytes for the     
  fixed header prefix. But look at find_max_id — the one place it should have been used:
                                                                                                                                                                             
  // Skip version and flags                                                                                                                                                
  file.seek(SeekFrom::Current(2))?;                                                                                                                                        
  // Read ID ...
  // Skip: sequence(8) + branch(8) + timestamp(8)
  file.seek(SeekFrom::Current(24))?;

  The seeks are interleaved with reads, so a single "header size" constant doesn't fit the access pattern. The constant was defined as documentation of the format, then
  never actually used because the real code does field-by-field I/O. Classic "define first, implement differently."

  RecordLog.path and RecordIndex.path — also born in the initial release. Both stored the path on construction, never read it back. Likely intended for future error messages
   ("corruption in {path}"), reopen-after-fsync, or debug logging — but none of those were ever implemented.

  Subscription.id — born in ad27d7d (subscription system). The id is stored inside the struct, but the struct is already keyed by that same id in a HashMap<SubscriptionId,
  Subscription>. The struct never needs to know its own id — every code path that needs it already has it from the map key or the SubscriptionHandle.

  The mut in sync() — sync_all() takes &self on File, not &mut self. The write lock is needed to exclude concurrent writers, but the guard itself doesn't need to be mutable.
   Meanwhile append() correctly uses mut because seek() and write_all() do need &mut self.

[Anarchid]

Added: e4484e2 Server: return empty results for registered-but-empty append_log states (fixes crash when querying a state key that was registered but has no records yet)