Step 20e/23 (Phase 6): CI — reduce Actions cache storage pressure without shrinking coverage

[Jordan231111]

Summary

Reduce GitHub Actions cache storage pressure and simplify the CI cache strategy without reducing any of the current cross-platform test coverage.

Related: #167, #173
Why this is post-MVP: the current CI matrix now covers the shipped x64/ARM targets across Linux, macOS, and Windows. This follow-up is about cache/storage efficiency and long-term CI maintainability, not MVP feature validation.

Key Changes

1. Audit the current overlapping cache layers (actions/cache, Swatinem/rust-cache, and sccache) and remove redundant persistence.
2. Decide whether sccache should be removed from CI entirely or limited to a smaller subset of lanes where it provides clear value.
3. Restrict cache saves to durable refs (for example main and release tags) where appropriate instead of creating unnecessary branch/PR cache churn.
4. Keep the native CI test matrix coverage unchanged for supported Linux/macOS/Windows x64 and ARM targets.
5. Document the chosen cache strategy in workflow comments so future changes do not accidentally recreate storage pressure.

Acceptance Criteria

• CI test coverage remains intact for the currently shipped Linux/macOS/Windows x64 and ARM release targets
• Redundant Cargo registry/git cache persistence is removed or explicitly justified
• Cache writes are limited to refs/jobs that provide durable value
• Repository cache usage no longer trends near the default GitHub Actions cache storage ceiling during normal development
• Workflow comments clearly describe the intended cache strategy and tradeoffs