fp-stability: split the 1876-line module into metrics/runners/report (no behavior change)

The PR had grown fp_stability.py well past the 1000-line soft guideline. Pure relocation — no function body, string, constant, or logic changed — into a clean dependency chain:

- fp_stability_metrics.py (474, leaf): regexes/constants + pure parsing, source-reading, sig-bits, cancellation, ranking, statement-bounds helpers. Imports no sibling.

- fp_stability_runners.py (530): Verrou subprocess orchestration (run/dd/vprec/cancellation/confirmation/disambiguation). Imports metrics.

- fp_stability_report.py (244): GitHub summary + annotation emitters. Imports metrics.

- fp_stability.py (715): CLI entry, CASES, _run_case, _blank_result; imports explicitly from the three. No import cycles.

Also dropped the unused 'case' parameter from _run_cancellation_check/_run_float_max_check. Verified: 69 toolchain tests, ruff (incl. F-rules: no undefined names), precheck all 7, and a live fp-stability run confirming the cross-module orchestration (sig-bits pass/fail + cancellation sweep) is unchanged. Test import repointed to fp_stability_metrics.

Author: sbryngelson