Add resplit_all_reject_merges() helper in phase_z2_composition.py that
detects parent_merged / parent_merged_inferred units with label=reject
and rebuilds them as per-section single units using each section's own
rank-1 V4 evidence (no frame swap, MDX raw_content preserved).
Pipeline hook fires once after Step 6 settling chain (u12/u4/empty-shell)
and section_assignment_plan resolution, before Step 6 artifact write.
Guards: beneficial-split rule (>=1 non-reject), coverage equality, layout
cap (>4 abort), max_retry=1, section_assignment_override short-circuit.
Audit: comp_debug["imp48_resplit"] additive payload (applied, split_units,
skipped_units, post_split_unit_count, post_split_layout_preset);
selection_path="resplit_from_merge" telemetry on rebuilt singles;
layout_preset re-derived via select_layout_preset(new_units).
Tests: 39/39 PASS (composition u1~u6: 14 cases; pipeline u7~u9: 25 cases).
Scoped regression 720/6 with 6 failures isolated as pre-existing on
baseline 79f9ea5 (independent of IMP-48). mdx03 golden lock preserved.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
