fix(partitions): account for segment-backed append batches

[jiengup]

Which issue does this PR address?

Closes #4142

Rationale

Group commit used the padded full prepare size even when segment storage wrote only a small reference record to the WAL. This prevented large segment-backed messages from coalescing despite their small WAL footprint.

What changed?

Append batching now tracks retained capacity, actual padded WAL extent, and segment-body work separately. The WAL extent controls group-commit coalescing, while an independent segment-body limit preserves the safeguard requested in the #4092 review discussion.

The previously ignored eight-message test now completes in one barrier group. A nine-message regression test verifies that batches still split when segment-body work exceeds its limit.

Local Execution

• Passed cargo fmt --all
• Passed cargo sort --no-format --workspace
• Passed cargo clippy -p journal -p partitions -p simulator --all-features --all-targets -- -D warnings
• Passed cargo test -p journal -p partitions -p simulator
• Passed ./scripts/ci/taplo.sh --check
• Pre-commit hooks ran and passed

AI Usage

1. Tool: OpenAI Codex
2. Scope: Repository analysis, implementation assistance, test updates, and review of the final diff.
3. Verification: Ran the relevant journal, partitions, and simulator tests, clippy, formatting, Taplo, and pre-commit checks.
4. I reviewed and can explain every line of the change.

[github-actions]

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See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 88.46154% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.57%. Comparing base (1284e30) to head (d3de54f).

Files with missing lines	Patch %	Lines
core/partitions/src/persistence.rs	89.55%	1 Missing and 6 partials ⚠️
core/journal/src/partition_journal.rs	81.81%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #4202       +/-   ##
=============================================
- Coverage     87.46%   70.57%   -16.89%     
  Complexity     1575     1575               
=============================================
  Files          1280     1278        -2     
  Lines        223192   197661    -25531     
  Branches     186557   161023    -25534     
=============================================
- Hits         195223   139509    -55714     
- Misses        23264    53535    +30271     
+ Partials       4705     4617       -88     

Components	Coverage Δ
Rust Core	67.64% <88.46%> (-20.89%)	⬇️
Java SDK	68.68% <ø> (ø)
C# SDK	77.48% <ø> (+0.02%)	⬆️
Python SDK	90.97% <ø> (ø)
PHP SDK	85.67% <ø> (ø)
Node SDK	96.45% <ø> (+0.02%)	⬆️
Go SDK	70.08% <ø> (-0.09%)	⬇️

Files with missing lines	Coverage Δ
core/journal/src/partition_journal.rs	94.60% <81.81%> (-0.06%)	⬇️
core/partitions/src/persistence.rs	91.88% <89.55%> (-0.16%)	⬇️

... and 356 files with indirect coverage changes

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