fix(partitions): sync checkpoints through original writers

[jiengup]

Which issue does this PR address?

Closes #4141
Relates to #4252

Rationale

Linux reports delayed writeback errors through per-open-file error
cursors. A checkpoint that reopens a materialized path after the error
can miss it and reclaim the WAL after the original writer lost bytes.

What changed?

Checkpoint mutations now carry durability barriers tied to the original
writers. Active indexes keep their existing synchronous fence and pass
an internal already-synced barrier, while simulator barriers retain the
original SimFile until the checkpoint mutation runs.

Every supplied barrier completes before path and directory barriers and
before WAL checkpoint publication. The simulator now covers the original
#4141 failure and multiple writers for one file, and records the related
install-backup gap from #4252 as an ignored follow-up regression.

Local Execution

Passed:

• cargo fmt --all
• cargo sort --no-format --workspace
• cargo check -p partitions --no-default-features
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test -p journal --lib
  • 95 passed, 0 failed
• cargo test -p partitions --lib
  • 304 passed, 0 failed, 3 ignored
• cargo test -p simulator --lib
  • 150 passed, 0 failed, 4 ignored
• ./scripts/ci/taplo.sh --check
• git diff --check

The #4141 regression was confirmed red before the implementation and
passes after the fix:

cargo test -p simulator --lib \
  given_a_failed_writeback_when_checkpointing_then_wal_history_should_not_be_reclaimed

The ignored #4252 regression was also confirmed to fail for the expected
reason: install_backup::link_tree publishes the backup after syncing a
fresh hard-link handle that sampled past the original writer's error.
#4252 works as a follow-up issue which will not block this PR.

AI Usage

1. Tool: OpenAI Codex through Delta.
2. Scope: root-cause analysis, checkpoint barrier design, implementation,
   deterministic regression tests, and repository-wide storage-path audit.
3. Verification: red-green regression runs, journal/partitions/simulator
   unit suites, full workspace Clippy, formatting, Cargo sorting, and TOML
   checks.
4. I can explain every changed line if asked.

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

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• /pin - exempt the PR from the stale bot, /unpin to undo

See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 97.77778% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.37%. Comparing base (fa80704) to head (5a62498).

Files with missing lines	Patch %	Lines
core/partitions/src/iggy_partition.rs	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #4253       +/-   ##
=============================================
- Coverage     87.59%   69.37%   -18.23%     
- Complexity     1575     1576        +1     
=============================================
  Files          1284     1282        -2     
  Lines        224493   191250    -33243     
  Branches     187856   154614    -33242     
=============================================
- Hits         196653   132676    -63977     
- Misses        23127    53964    +30837     
+ Partials       4713     4610      -103     

Components	Coverage Δ
Rust Core	66.01% <97.77%> (-22.69%)	⬇️
Java SDK	68.70% <ø> (+0.01%)	⬆️
C# SDK	77.42% <ø> (+0.07%)	⬆️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.43% <ø> (+0.06%)	⬆️
Go SDK	70.11% <ø> (+0.05%)	⬆️

Files with missing lines	Coverage Δ
core/partitions/src/iggy_index_writer.rs	93.81% <100.00%> (+0.19%)	⬆️
core/partitions/src/lib.rs	0.00% <ø> (ø)
core/partitions/src/persistence.rs	92.05% <100.00%> (+0.17%)	⬆️
core/partitions/src/iggy_partition.rs	92.90% <92.30%> (+<0.01%)	⬆️

... and 369 files with indirect coverage changes

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[jiengup]

/request-review @numinnex