Add semiprime-window procedural heuristic and expanded-meta instrumentation

[GISMO-1]

Motivation

• Reduce residual expanded-case pressure caused by semiprimes that the existing prime/square heuristics miss by deterministically widening the procedural window for semiprime candidates.
• Use a cheap, deterministic gating test (residue + bounded SPF) so the heuristic is cheap and auditable.
• Provide instrumentation in exported expanded-meta so coverage and trigger behavior can be measured precisely.

Description

• Add semiprime_window_trigger(n, bound) helper that checks n % 48 in {1,25}, computes a bounded SPF, and returns (triggered, spf, bound_used) in src/strausforge/factor_meta.py.
• Introduce new procedural heuristic option semiprime-window and wire it through the code paths and CLI choices (PROC_HEURISTIC_CHOICES, help text, and default heuristic selection).
• Make procedural evaluation accept a semiprime_factor_bound and, when semiprime-window triggers, widen the initial procedural window deterministically (currently 8 -> 128) in src/strausforge/identities.py.
• Default the SPF trial bound used for expanded metadata to 5000 (was 20000) while keeping the CLI --expanded-factor-bound override, and pass that bound into procedural evaluation in src/strausforge/hardness_core.py.
• Enrich --export-expanded-meta records with semiprime_triggered: bool and spf_bound_used: int so trigger coverage is explicit in exported JSONL.
• Update CLI defaults/help and README.md documentation to describe the heuristic and the new default bound.
• Add unit/integration tests covering the trigger helper, metadata fields, and heuristic behavior in tests/test_factor_meta.py and tests/test_identities_verify.py.

Testing

• Installed dev deps and ran python -m pip install -e '.[dev]' successfully.
• Ran focused unit tests with python -m pytest tests/test_factor_meta.py tests/test_identities_verify.py -q and python -m pytest tests/test_cli.py -q -k "proc_heuristic or expanded_meta"; the new and affected tests passed.
• Ran additional focused suite python -m pytest tests/test_factor_meta.py tests/test_identities_verify.py tests/test_cli.py -q -k "proc_heuristic or factor_meta or identities_verify" and linters with python -m ruff check ...; all invoked tests and ruff checks passed.
• Committed the changes after tests/linting succeeded.

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Codex Task