perf(python): climb the walk cursor for scope-directive and default checks

[Fieldnote-Echo]

Fixes #2417

What does this PR do?

For nearly every Python identifier, usage extraction climbs toward the root with ts_node_parent, looking for a global/nonlocal ancestor (lexical_ancestor_kind, internal/cbm/extract_usages.c:1883) and for a parameter default (python_default_value_reference, :1782). Each ts_node_parent call descends from the root again, so an expression N terms deep costs O(N³). On error-free trees the climbs now follow the walk's occurrence cursor (O(1) per hop, as #2352 did for ReScript). For global/nonlocal one parent step is enough, because the grammar makes those names direct children of the statement. Trees with errors keep today's climb. The graph is byte-identical on 8 corpora, error-recovery trees included.

Linux x86_64, 32 logical CPUs, GCC 16	main	this PR
Definitions pass, generated r = c0*x + …, N = 1000 / 2000	16.8 s / unfinished at 90 s	40 / 150 ms
Definitions pass, sympy 1.14 resolvent_lookup.py alone	8.7-8.8 s	53-54 ms
parallel_extract median, 823 / 2063 ordinary .py files	1.54 / 4.75 s	1.15 / 3.58 s

extract_python_deep_default_parent_climbs_are_linear counts root-descending parent steps at 200 and 400 terms: 0 here, 62,544 and 245,044 on main with the same counter wired in. Unmodified main passes it, because its climbs do not report to the counter. repro_python_scope_directives_bind_the_named_scope covers the global/nonlocal binding.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
  (run as make -f Makefile.cbm test-par)
• Lint passes (make -f Makefile.cbm lint-ci)
  (lint-ci stops at cppcheck with the same findings as on main, none in files touched here; lint-format, lint-no-suppress and lint-memory-core pass)
• New behavior is covered by a test (reproduce-first for bug fixes)

Prepared with AI assistance (exploration, reproduction, verification, implementation); the spec and design decisions are mine, and I reviewed and ran everything above.

[github-actions]

Thanks for opening this — it has been seen, and it is queued.

This note is automated, but it is not a brush-off: it exists so you know where your PR stands instead of having to guess from silence.

Current review status: working through a backlog. 0.9.1-rc.1 is out, so the release freeze that held reviews is over — but it left a large queue of open pull requests behind it, and we are reading through them oldest-first. The background is in discussion #1144.

What that means for this PR, concretely:

• It will not be closed for inactivity. No stale bot touches pull requests here.
• It may still sit a while before a human reads it. That is on us, not on you.
• Older PRs are read first, so a recent one is not being skipped — it is behind a queue.

Things that will genuinely speed it up whenever review does happen:

• Keep it rebased on main — the tree is moving quickly right now, and a conflicting branch cannot be reviewed as the diff you intended.
• Get CI green, or say which failures you believe are pre-existing.
• Keep the change to one claim. Bundled features and refactors get split before they get merged, which costs you a round trip.
• Every commit needs a sign-off (git commit -s) — CI enforces DCO.

If this fixes a bug, a reproduction we can run is worth more than a description of the symptom.

Thanks for contributing, and sorry in advance for the wait.