fix(I-24): parity_block.py — a zero or empty COMPARATOR band is a named refusal, never a ZeroDivisionError traceback; case table both polarities wired into guard-runner-labels (PMAT-972, #2887, #2735)

[noahgift]

PP-066 DAG row I-24 (issue #2887; defect #2735; epic #2873; ticket PMAT-972; owner perf-gate; receipt docs/audits/impl-PMAT-972-receipt.md, status: partial until merged).

The defect (#2735, verified on main): a zero-token SUBJECT band refused cleanly, a zero-token COMPARATOR band reached median(subject) / median(comparator) and died with a Python ZeroDivisionError traceback — a crash where a verdict belongs.

What lands: _comparator_median(values, label) in scripts/lib/parity_block.py — every median-as-divisor site goes through it; an empty comparator band refuses band <c> comparator: no samples is not a measurement, a non-positive median refuses band <c> comparator: non-positive rate is not a measurement, both through the script's existing refusal path (exit 1); the subject refusal is untouched. scripts/check_parity_block_refusals.sh --selftest (RED first at 72383a9): four generated fixtures — control rc 0; zero comparator, zero subject, empty comparator rc 1 with their texts — every row also asserting no Traceback; wired into guard-runner-labels. Contract contracts/apr-parity-block-refusals-v1.yaml (kind: pattern); README 1812 contracts. _executor_side/_executor_lane/build/main decomposed by pure code motion because the pre-commit complexity gate refused the file over debt three of them carried on main already.

Acceptance, re-run by the orchestrator

A_i	rc
bash scripts/check_parity_block_refusals.sh --selftest (4 fixtures)	PASS
python3 -m py_compile scripts/lib/parity_block.py · bash scripts/check_thresholds_in_matrix.sh --selftest	0 · 6 passed
pv validate · pv lint · check_contract_test_binding.sh · check_contract_enforcement.sh · check_readme_claims.sh · check_no_claim_literals.sh · check_roadmap_diff_additive.sh · check_guards_are_wired.sh · check_workflow_env_defined.sh · bashrs lint (0 errors)	0 each

Mutation — RED, then restored GREEN: the guard call and the guarded division replaced by the bare statistics.median(subject_values) / statistics.median(comparator_values) → row 2 FAILED with ZeroDivisionError: float division by zero and contains forbidden text: Traceback; restored → PASS.

Recorded: a first docstring (0 / positive == 0.0) read as a float comparison to check_thresholds_in_matrix.sh's CMP regex and broke its self-test — reworded; the card's mutation line numbers had drifted, the sites are now one helper. Receipt for this PR itself: advisory, not produced (driver A1).

[github-actions]

§13.11 rung 1 — quorum shadow verdict

S13-SHADOW pr=3006 head=131de38a2daf34b3815e67c0e565a57f81c73dfb verdict=REFUSE class=Q1 arm_rc=1

Shadow mode: this records a verdict and merges nothing. A refusal
to arm is not a block (§13 adds zero rows to §7) — the pull request is
exactly as green as it was.