F-089: Safe Args navigation task cache (GH #110)

[stepango]

Summary

• Closes the GH Navigation task cache is broken #110 / F-089 navigation Safe Args build-cache gap by verifying current toolchain behavior and documenting the regression.
• Root cause was historical: ArgumentsGenerationTask.navigationFiles used absolute path sensitivity; Navigation Safe Args 2.9.8 (sample pin) already uses @PathSensitive(RELATIVE) + @CacheableTask.
• No Forma plugin fork required — Path B navigationRes + existing pin is the fix surface.

Verify (host)

• Two worktrees + shared local build cache: :core-navigation-res:generateSafeArgsDebug same cache key → second location FROM-CACHE
• Main tree clean → FROM-CACHE; warm → UP-TO-DATE
• --configuration-cache build successful for the task

Docs

• docs/CONFIGURATION-PERFORMANCE.md — Safe Args task cache section + re-check recipe
• docs/TARGET-PLUGINS.md — pin pointer
• TICKETS.md / docs/PROGRESS.md — F-089 done

Test plan

• Bytecode check of safe-args plugin jars (2.7.4–2.9.8) for RELATIVE sensitivity
• Two-location build-cache experiment on sample navigationRes
• CI green on this PR

Closes #110