[den-run-ai]

Fixes #22370.

Reuse the external signature index across C modules within one stubgen invocation. Keep loading lazy and scoped to the invocation so later runs pick up documentation changes.

With four C modules and 319 documentation files, file reads fall from 1,276 to 319. Benchmark details are in the issue.

Add a regression test covering reuse across modules and documentation changes between invocations.

Validation: 379 stubgen tests passed (1 skipped, 2 xfailed); serial self-check (196 files); Black and Ruff.

[agammann]

I checked the loader’s lifetime and call paths. The cached loader is created inside generate_stubs, so C modules in one run share the parsed documentation index while a later invocation reloads changed docs. Direct callers of generate_stub_for_c_module still use its uncached default. The two-module/two-invocation test covers both reuse and invalidation. I did not run the full suite.