* fix: zero-length file commit panic and stale missing-file carry-forward

Two related bugs in the checkin path:

#68 - Committing a zero-length file panicked in blob.Store, which treated an empty artifact (a normal, well-known Fossil blob) as invalid input. Worse, manifest.Checkin's postcondition defer fired unconditionally during the panic's stack unwind and re-panicked with its own unrelated "manifestRid must be positive" message, masking the real cause. Fixed both: blob.Store now accepts zero-length (and nil) content, and Checkin's defer re-panics with the original panic value instead of asserting a postcondition that was never going to hold mid-unwind. Fixing the panic surfaced a second bug in the same family: buildCommitFiles used len(Content) > 0 to detect "already loaded from disk," which can't distinguish a genuinely empty file from one not yet loaded, so a newly added empty file failed with "no RID for unchanged file." Replaced the length sentinel with an explicit loaded-set.

#79 - A tracked file deleted from disk without Unmanage/Remove was silently carried forward into the new manifest with its last-committed content, rather than being detected. Verified against fossil 2.28: canonical fails a commit when a missing tracked file is in scope (either the implicit "commit everything" case, or explicitly named), but leaves an out-of-scope missing file alone. buildCommitFiles now stats every in-scope tracked file before treating it as unchanged and errors clearly, naming the file, when it's gone; a file outside the commit's scope (relevant only to an explicit Enqueue) is left untouched exactly as before. Updated the two existing tests that had documented the old silent-carry-forward behavior as deliberately deferred to this issue.

* fix: add CHANGELOG entry and unwrap-safe stat checks for #68/#79

Adds the CHANGELOG entries the #79 acceptance criteria required for a user-visible behavior change (missing tracked file now hard-fails a commit instead of silently carrying stale content forward) and for the #68 zero-length-file panic fix.

Also switches checkin.go's two os.IsNotExist(err) checks to errors.Is(err, os.ErrNotExist), which correctly unwraps the %w-wrapped os.ErrNotExist that MemStorage.Stat returns. Production was already correct (OSStorage.Stat returns a raw *PathError), but the tests were only exercising a generic fallback error branch instead of the intended "tracked file missing from disk" message. check-in: 0351167ae5 user: [email protected] tags: trunk