test: add cache safety assertions for diff operations

[rocketman-code]

Context

Parent: #160

The showstopper v0.4.0 bug (#142) was diff silently corrupting the disk cache. build_snapshot_from_working_tree() called load_graph(entry, false) — caching enabled — which wrote a reduced git-tree graph to .chainsaw.cache, overwriting the full graph. Subsequent trace/packages returned wrong results.

Expected Behavior

Tests that verify diff operations never read or write the disk cache.

Scope

• Test: run trace (builds cache), then diff HEAD~1, then trace again — results must match
• Test: after diff, .chainsaw.cache mtime is unchanged
• Test: build_snapshot_from_ref() and build_snapshot_from_working_tree() don't touch cache
• Consider a test helper that wraps cache operations with a sentinel file to detect unexpected writes
• Consider a #[cfg(test)] assertion in load_graph that panics if called with no_cache=false during a diff context