feat: cxx_scan restat + multi-fingerprint build cache LRU (P2+P3)#37
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P3: .build_cache upgraded from single-entry to multi-entry format. Each entry is keyed by target triple (e.g. "" for default, "musl" for --target). Up to 4 entries (LRU). Switching between default and musl targets no longer invalidates each other's fast-path cache. Legacy single-entry format is auto-detected and migrated on read. P2 (cxx_scan restat) deferred: the backup-compare-restore approach breaks CI self-host smoke tests where cached target/ artifacts from previous runs interact with ninja's restat logic. The existing restat on cxx_dyndep and cxx_module rules already prevents most cascading rebuilds, making P2's marginal benefit not worth the risk.
Sunrisepeak
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Summary
restat = 1, downstream edges are not dirtied.touch src/ui.cppmnow runs 6 edges (scan + direct dependents + link) instead of 40+..build_cacheupgraded to multi-entry format keyed by target triple. Up to 4 entries. Switching between default/musl targets no longer invalidates each other's fast-path. Legacy format auto-detected and migrated.Test plan
mcpp build+mcpp test— 12/12 pass.build_cachewritten in new[target=]section format.build_cachecorrectly read (backward compat)touch src/ui.cppm→ ninja runs 6 edges (not 40+) via restat