fix(resident): report unflushed changes after crash

[y4ho0]

Summary

• write a small durable marker before acknowledging the first resident mutation that is still only in memory
• clear the marker after every successful save, autosave, flush barrier, rollback, or normal shutdown
• surface a one-shot warning when the next resident reopens the last saved file after an abnormal exit
• make save and close return resident_unflushed_changes_lost instead of claiming the stale file is already saved

Scope

This implements the issue's minimum honest-recovery option. It does not replay lost edits: replaying arbitrary resident operations as a WAL can duplicate non-idempotent mutations if a process dies after saving the document but before clearing the journal. The warning instead tells the caller that the resident may have lost acknowledged in-memory edits and points short-lived agent harnesses to OFFICECLI_RESIDENT_FLUSH=each.

The marker contains only a version, process ID, and timestamp. Its filename is derived from the same canonical per-document identity used by resident IPC; it does not store the document path.

Validation

• dotnet build src/officecli/officecli.csproj -c Release --no-restore (0 errors; one pre-existing nullable warning in ExcelHandler.SheetShift.cs)
• controlled deferred edit + validated resident PID + SIGKILL: memory held after crash, disk held before crash, and the next save --json exited 1 with resident_unflushed_changes_lost
• explicit save control + validated resident PID + SIGKILL: disk retained saved value, and the next save --json succeeded without a recovery warning
• reopen control: the first get --json returned the last saved value with the recovery warning; the second returned the value without repeating the warning
• git diff --check

Refs #328