T19-01 Phase 1: Reproduction contract and failing tests

[edithatogo]

Track phase issue governed by the T19-01 Conductor plan.

Parent track: #550
Parent phase: #554
Plan: conductor/tracks/T19-01/plan.md

Implementation is not authorized by planning-only track creation.

• Freeze privacy-safe fixtures for full-editor rich content, restricted-editor incompatibilities and primary-editor table incompatibility.
• Add failing unit tests for exact JSON-path and node-type diagnostics.
• Add failing contract tests that keep public rendering, primary compatibility, auxiliary risk and browser verification distinct.
• Add a deterministic reproduction for duplicate editor mounting and DOM-cost metrics.
• Phase Verification & Checkpoint (see conductor/workflow.md).

[edithatogo]

Season 1 evidence update: the reproduction fixture must cover six independent scheduled posts with primary-editor tableHeader failures (counts 7, 7, 4, 4, 8, 11), plus a paragraph-only passing cover control and the separate rich published-pilot control. See the audit comment on #550.

[edithatogo]

Completed via PR #601. Frozen fixtures for cover control, rich pilot control, and me-001..me-006 created with 11 tables and 41 tableHeader nodes.