fix(db): surface disk-full staging errors

[corylanou]

Description

Adds a typed LTXStagingDiskFullError plus ErrDiskFull for local LTX staging failures, and wraps ENOSPC/EDQUOT from the L0 staging open/write/encoder close/fsync/close path before it reaches the generic sync loop.

The DB monitor now logs a clear sync stopped: disk full while staging ltx file error with the staging path and LTX identity, then exits instead of retrying the same generic sync error forever.

This PR is stacked on the issue-1164-soak-replicate-restore-test stability cluster branch.

Motivation and Context

Issue #1310 reports v0.5.x initial sync staging the full snapshot under .{db}-litestream/ltx/0/ before upload. In the reported case, a 22 GB database with about 20 GB free filled the disk during .ltx.tmp creation, Litestream stayed active, no object reached the replica, and the journal had no clear ERROR/WARN signal.

This PR is intentionally scoped to surfacing and fail-fast handling of the local disk-full staging condition. It does not change the snapshot staging architecture or stream initial snapshots directly to the replica.

Fixes #1310

How Has This Been Tested?

• go test -run TestDB_SyncReturnsDiskFullErrorForLTXStaging ./...
• go build ./...
• go test -race ./...
• pre-commit run --all-files

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	✅	36.98 MB (+8.0 KB / +0.02%)
Dependencies	✅	No changes
Vulnerabilities	✅	None detected
Go toolchain	✅	1.25.12 (latest)
Module graph	✅	1240 edges (0)

Binary Size

	Size	Change
Base (906619b)	36.98 MB
PR (14a4dac)	36.98 MB	+8.0 KB (+0.02%)

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	32s
Go version	go1.25.12
Commit	14a4dac

History (8 previous)

Commit	Updated	Status	Summary
1c098fc	2026-07-16 15:08 UTC	✅	36.98 MB (+8.0 KB / +0.02%)
c7488a1	2026-07-08 20:45 UTC	✅	36.94 MB (+4.0 KB / +0.01%)
05faeb0	2026-07-08 20:40 UTC	✅	36.94 MB (+8.0 KB / +0.02%)
0282358	2026-07-08 18:54 UTC	✅	36.94 MB (+8.0 KB / +0.02%)
b814779	2026-07-08 18:50 UTC	✅	36.94 MB (+8.0 KB / +0.02%)
2f795f5	2026-07-02 03:44 UTC	✅	36.93 MB (+8.0 KB / +0.02%)
d361760	2026-06-26 15:06 UTC	✅	36.93 MB (+12.0 KB / +0.03%)
0ae9081	2026-06-25 01:59 UTC	✅	36.93 MB (+8.0 KB / +0.02%)

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_{🤖 Updated on each push.}