build: Red Bear cache system — resilient to make clean

Adds comprehensive build cache snapshot and restore for overlay OS.

Problem: Upstream Redox build system is single-stream — make clean
destroys cached stage.pkgar files permanently. Build can't recover
without full from-scratch rebuild (2-4 hours).

Solution: Red Bear cache system provides:
- snapshot-cache.sh: Save all stage.pkgar to local/cache/
- restore-cache.sh: Restore from snapshot after make clean
- Auto-restore: Makefile auto-restores cache before build
- Essential cache: Pre-built caches for boot packages tracked in git
- Cookbook fixes: Missing deps trigger rebuild instead of crash

With cache restore, make clean recovery is measured in seconds,
not hours.

Gaps fixed in cookbook:
- modified_all_btree: missing dep → UNIX_EPOCH (rebuild trigger)
- sysroot install: missing dep → skip + rebuild

local/cache/.gitkeep

@@ -0,0 +1 @@
+# Red Bear cache — essential packages tracked, full snapshots gitignored