fix(replica): snapshot missing local L0 gaps

[corylanou]

Description

When L0 gap healing finds that the missing remote range is also absent locally, force a snapshot at the current database position and advance the replica position to the snapshot TXID. Position recalculation recognizes a snapshot that covers the gap, so later compaction invalidations do not force duplicate snapshots.

Latest restore planning starts from the forced snapshot and does not select the gapped L0 history below it. Other local-open errors and backend write errors retain their existing failure behavior.

This PR stacks on #1462, which stacks on #1155. Its base must remain issue-1457-gap-heal-upload-only-missing until the stack lands.

Motivation and Context

The regression test reproduced the failure before the fix: deleting a two-TXID range from remote L0 and its matching local files caused Replica.Sync to fail at the first local open with no such file or directory. Position cleanup then made every retry repeat the same failure.

The fallback is limited to os.IsNotExist from the expected local L0 path while a later remote L0 file proves an interior gap. It emits a WARN with the full gap range, creates one current-position snapshot, and resumes replication above it.

Fixes #1458

Related: #1462, #1155

How Has This Been Tested?

• TDD regression verified the missing local L0 open failed before the production change.
• GOTOOLCHAIN=go1.25.13 go test -race -run "^(TestReplica_InvalidatePos_(HealsL0Gap|MissingLocalL0FallsBackToSnapshot)|TestDB_Compact/L0GapHealsViaReplica|TestReplica_CalcRestorePlan)$" -count=1 .
• GOTOOLCHAIN=go1.25.13 go test -race ./...
• GOTOOLCHAIN=go1.25.13 pre-commit run --all-files
• LOG_LEVEL=DEBUG GOTOOLCHAIN=go1.25.13 go test -tags "integration,soak" -run "^TestLTXBehavior$" -short -v -timeout 15m ./tests/integration/
• The regression restores a real SQLite database after fallback, verifies its rows, confirms a snapshot-only restore plan, invalidates the position again without another snapshot, and verifies a subsequent transaction replicates.
• Low-, high-, and burst-volume behavioral profiles passed all applicable assertions and restore integrity checks.

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

⚠️ Attention needed — Go toolchain outdated (1.25.13 → 1.25.14)

Check	Status	Summary
Binary size	✅	37.17 MB (0.0 KB / 0.00%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	⚠️	1.25.13 → 1.25.14 available
Module graph	✅	1230 edges (0)

Binary Size

	Size	Change
Base (b53e3ea)	37.17 MB
PR (6aa33c0)	37.17 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 1 vulnerability in packages you import and 3
vulnerabilities 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	31s
Go version	go1.25.13
Commit	6aa33c0

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_{🤖 Updated on each push.}