Pin that selected verification calls back through a plain function

[Deicyde]

Summary

Follow-up from the post-merge review of #307 (#307 (comment)): one small test, no code change.

• Slim the Lean capture code and tests #257 added, without changing what they pin #307 passes verify_recorded to _recheck_scanned_directory as a callback. A lambda there adds a Python frame per folder level, which test_selected_verification_recurses_in_three_frames_per_folder already pins. A C-implemented wrapper such as functools.partial adds no Python frame but spends CPython's C recursion budget at every level, so deep Lean trees fail with "nested too deeply" where main captured them. fd731bd removed the partial; nothing caught putting it back.
• test_selected_verification_calls_back_through_a_plain_function records the type of every verify_kept callback in a Lean capture and requires a plain Python function.

Tests

• The new test fails with verify_kept=functools.partial(verify_recorded) restored and passes on main; the frame test passes either way, which is the gap.
• tests/test_lean_sources.py and tests/test_tree_snapshot_limits.py pass on Python 3.13; the new test passes on 3.10. ruff check is clean.

[Deicyde]

Merge readiness: ready to merge (reviewed at e82523d)

• CI is green on every job. On the Linux test jobs the new test runs, not skips: one more pass than main, skips unchanged.
• An independent review mutated the callback:
  • The new test fails for every C-implemented callable it tried: functools.partial(verify_recorded), the C method-wrapper verify_recorded.__call__, an lru_cache-wrapped function and a staticmethod. It passes with the plain callback on main, on Python 3.13 and 3.10.
  • It does not interfere with test_selected_verification_recurses_in_three_frames_per_folder: 30 runs in both orders all passed.
  • It is stable under pytest-randomly (4 seeds), and it skips correctly where descriptor capture is unavailable.
• Worth its lines: 26 lines guard a regression that Slim the Lean capture code and tests #257 added, without changing what they pin #307's review history introduced once and that the frame test cannot see.

Optional: the body says a C-implemented wrapper makes deep trees fail. That only happens on CPython 3.12 and later, with a raised recursion limit or little C budget left at the call site.

It is a draft, so it needs to be marked ready before merging.