[codex] Stabilize incremental rebuild caching

[Sunrisepeak]

Summary

• Change BMI cache staging to only copy missing cached artifacts into the project target directory; existing .gcm/.o outputs are no longer overwritten or touched.
• Add normalized compiler driver identity (Toolchain::driverIdent) and use it for fingerprint field 3 instead of hashing the compiler binary contents when available.
• Normalize local install path fragments in compiler --version output so equivalent xlings toolchains installed under different prefixes share a fingerprint.
• Add regression coverage for BMI staging, driver output normalization, toolchain detection, and fingerprint stability.
• Add .agents/docs/2026-05-15-fingerprint-stability-and-fastpath-coherence.md with the PR comparison and follow-up design notes.

Root Cause

There were two related incremental rebuild problems:

1. BMI cache staging copied cached dependency artifacts into the active target directory on every full prepare and refreshed mtimes. That dirtied downstream modules after ordinary source edits.
2. Fingerprint field 3 used hash_file(tc.binaryPath). Equivalent xlings GCC packages installed under different registry prefixes can have different binary bytes, so switching between local/dev mcpp and installed mcpp could silently select a new target/<triple>/<fingerprint> directory and look like a full rebuild.

Validation

• target/x86_64-linux-gnu/b2f2305667355830/bin/test_bmi_cache
• target/x86_64-linux-gnu/b2f2305667355830/bin/test_toolchain_detect
• target/x86_64-linux-gnu/b2f2305667355830/bin/test_fingerprint
• git diff --check -- <changed files>
• mcpp build --verbose no-change path in the new fingerprint dir: ninja: no work to do
• touch src/main.cpp && mcpp build --verbose: 4 Ninja edges, 0.36s locally

Scope Notes

This PR intentionally does not change .build_cache format or cxx_scan restat behavior. Those are documented as follow-up P1/P2 items in the analysis doc so the 0.0.15 fix stays focused.