Prevent unnecessary rewrites in generated outputs

[novr]

Summary

• Introduce GeneratedFileWriter.writeIfChanged and route all Kawarimi generated output writes through it.
• Skip rewriting files when generated content is unchanged so output mtimes remain stable for incremental builds.
• Add unit tests for writeIfChanged behavior and update KawarimiTests target dependency for direct function testing.

Test plan

• swift test --filter KawarimiTests
• Verify writeIfChanged skips write on identical content
• Verify writeIfChanged overwrites file when content differs

[github-actions]

Kawarimi [kawarimi-perf] — small fixture

Commit ae20725 · Kawarimi writes under basename kawarimi-perf-out (see workflow RUNNER_TEMP).

[kawarimi-perf] phase=setup seconds=0.006157
[kawarimi-perf] phase=load seconds=0.015813
[kawarimi-perf] phase=generate_kawarimi seconds=0.001141
[kawarimi-perf] phase=generate_handler seconds=0.003751
[kawarimi-perf] phase=generate_spec seconds=0.003279
[kawarimi-perf] phase=total seconds=0.030450