fix(replica): fsync directory after durable renames

[corylanou]

Description

Adds internal.FsyncDir and calls it after every rename that finalizes a durable file:

• both Replica.Restore output paths (main + V3),
• local L0 LTX staging in db.go,
• the file replica backend's WriteLTXFile.

WriteTXIDFile's existing inline directory sync is refactored onto the same helper, so the crash-durability pattern (temp + fsync + rename + dir-sync) now lives in one place.

Motivation and Context

Without a directory fsync, a crash shortly after a successful os.Rename can lose the directory entry — a restore that reported success can leave no database file, and the file backend can lose a replica LTX object it reported as written. WriteTXIDFile already did this correctly; the other rename sites did not. The two non-restore sites were found by review while fixing the restore path (same defect class).

Fixes #1342

How Has This Been Tested?

go build ./..., go vet ./..., and go test -race for the root, file, and internal packages all pass. Durability-on-crash itself is not directly testable without fault injection; the change is the established WriteTXIDFile pattern applied to the remaining sites.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (would cause existing functionality to not work as expected)

Checklist

• My code follows the code style of this project (go fmt, go vet)
• I have tested my changes (go test ./...)
• I have updated the documentation accordingly (if needed)

[github-actions]

PR Build Metrics

✅ All clear — no issues detected

Check	Status	Summary
Binary size	✅	36.95 MB (0.0 KB / 0.00%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	✅	1.25.12 (latest)
Module graph	✅	1240 edges (0)

Binary Size

	Size	Change
Base (c96c0f4)	36.95 MB
PR (96711bc)	36.95 MB	0.0 KB (0.00%)

Dependency Changes

No dependency changes.

govulncheck Output

=== Symbol Results ===

No vulnerabilities found.

Your code is affected by 0 vulnerabilities.
This scan also found 0 vulnerabilities in packages you import and 1
vulnerability in modules you require, but your code doesn't appear to call these
vulnerabilities.
Use '-show verbose' for more details.

Build Info

Metric	Value
Build time	42s
Go version	go1.25.12
Commit	96711bc

---

_{🤖 Updated on each push.}