Comprehensive 6-tier plan to address the 1.5h full-rebuild pathology
when making small config changes. Covers content-hash output
fingerprinting, per-crate granularity, public API surface tracking,
restat / equivalence caching, and developer-experience tools.
Synthesizes techniques from Nix, Buildroot, Yocto, GN/Ninja, Cargo,
and Bazel adapted to Red Bear OS's Rust cookbook.
Triggered by: 2-line edit to local/sources/base/Cargo.toml caused
1.5h full rebuild of redbear-mini. Root cause: cookbook tracks at
recipe granularity (one stage.pkgar for 45-member Cargo workspace)
instead of crate granularity.
Red Bear CI