epic(cc): transparent compile-to-link cache coverage

[jleni]

Goal

Make Kache a practical, transparent C/C++ compiler cache: ordinary build-system recompiles remain cacheable, compiler-name shims can intercept builds without per-project configuration, and opt-in link caching safely covers the largest remaining passthrough class.

Active workstreams

• Existing output path check from #664 also disables caching for ordinary object compiles #744 — restore caching for ordinary recompiles whose output file already exists
• Support compiler-name shims on PATH for transparent, config-free interception #310 — support compiler-name shims on PATH without recursion
• Linker caching: cache whole-program link outputs (the largest passthrough category) #259 — cache whole-program link outputs and their complete sidecar set

Delivery order

1. Fix Existing output path check from #664 also disables caching for ordinary object compiles #744 without regressing special-file and symlink write semantics.
2. Add the transparent compiler entry point in Support compiler-name shims on PATH for transparent, config-free interception #310.
3. Build opt-in link caching in Linker caching: cache whole-program link outputs (the largest passthrough category) #259 after determinism and portability gates are proven.

Completed context

#337 established compiler-driver conformance, launcher scenarios, structured passthrough reasons and coverage gates. It remains closed; this epic tracks only the concrete remaining product path.

Exit criteria

• CMake configuration changes can recompile into existing regular object paths while still reading from and writing to Kache.
• /dev/null, device, FIFO, directory and symlink behavior remains compatible with the real compiler.
• A POSIX compiler-name shim resolves and invokes the real compiler without re-entering Kache; unsupported platforms and missing compilers fail clearly.
• Link keys cover ordered objects, archives, libraries, response files, linker flags and linker identity.
• Binary, import-library, map and debug sidecars are stored and restored atomically.
• Link caching stays opt-in until path portability and reproducibility negative controls pass.
• Representative isolated-checkout CMake/Firefox scenarios demonstrate object and link hits without false hits or path leakage.

Non-goals

• A persistent C/C++ preprocess memo from perf(cc): memoize preprocessor expansion hash for warm C/C++ hits #562 until workload evidence and an invalidation proof exist.
• Remote C/C++ sharing.
• General declarative bypass policy from feat(config): declarative bypass/refuse rules (argv pattern / env / crate) #222.
• Unsupported compiler families without a reproduced fixture.