Add test coverage for cache staleness detection functions

[Copilot]

The staleness detection functions (is_layer_stale, is_base_layer_stale, is_branch_layer_stale, is_working_layer_stale) lacked test coverage for critical cache invalidation logic.

Changes

Basic staleness scenarios:

• Missing cached layers
• Missing indexed SHA metadata
• Correct dispatch to layer-specific staleness checks

Working layer staleness:

• File modification time vs. cache timestamp
• Deleted/missing tracked files
• Empty file tracking

Git integration scenarios:

• Base layer: HEAD movement on main/master
• Branch layer: HEAD movement on feature branch
• Branch layer: merge-base changes after rebase

Edge cases:

• Missing git references (no commits)
• Detached HEAD state
• Layered index staleness propagation

All tests use real git repositories to validate actual git workflows (commits, branches, rebases) rather than mocking.

Example

#[test]
fn test_is_branch_layer_stale_merge_base_changed() {
    // Create feature branch from main
    // Make commits on both branches
    // Verify layer is not stale (merge-base unchanged)
    
    // Rebase feature onto main
    // Verify layer is stale (HEAD SHA changed)
}

15 new tests added, all passing (162 total).

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