[ENH](wal3): Type append contention

[domenkozar]

Description

Expose append contention as a structured AppendOutcome instead of requiring callers to interpret LogContention* errors as commit status.

The additive append_with_options_outcome API distinguishes:

• committed appends
• durable publication contention
• retryable contention
• indeterminate durability

Non-contention failures remain in the existing error path, and the legacy append API remains unchanged. This change applies directly to main and does not depend on #7644 or #7645.

Testing

• cargo test -p wal3 append_result_preserves_contention_semantics --lib
• Casita RustFS racing-writer and racing-initializer integration tests

[github-actions]

Reviewer Checklist

Please leverage this checklist to ensure your code review is thorough before approving

Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)

[claude]

Claude Code Review

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[domenkozar]

cc @rescrv