TypedPython: cache the generated C, not the built extension, once the pypackpack compile slot builds it #113

Today incremental.BuildCache stores the built .so (cbuild is the compile callable) and its key includes toolchain flags. With pypackpack's compile slot (pypackpack #33) compiling C to extensions, the two layers split: TypedPython caches source→C (key: source + dependency interfaces + COMPILER_VERSION + flags that change the C), and the slot caches C→extension in its buildDir.

Done when: compile_project can emit .c artefacts (one per compiled module, plus the runtime header dir) for the slot, the cache hits on an unchanged module with a byte-identical .c, and the existing in-process build path keeps working for tests. Blocked on the slot's interface settling (pypackpack #33).