T19-01 Phase 5: Upstream escalation and release evidence

[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.

• Generate a minimal synthetic reproduction package for Substack.
• Run unit, integration, end-to-end, smoke, mutation, property-based, autonomous-agent, metamorphic and consumer-contract coverage required by repository policy.
• Run type, formatting, contract, governance, security and diff checks.
• Record verification receipts and update issue T19-01 Published-post editor schema compatibility and crash containment #550 with reproducible evidence.
• Open a focused pull request and stop for review before runtime rollout.
• Phase Verification & Checkpoint (see conductor/workflow.md).

[edithatogo]

Upstream escalation evidence is now stronger: a scheduled essay still raises generic Tiptap Invalid JSON content after every table node is removed, while the paragraph-only season cover opens. The minimal upstream fixture should isolate image, heading, list and blockquote nodes in the auxiliary editor. See #550.

[edithatogo]

Completed via PR #601. Upstream reproduction package generation implemented with synthetic ProseMirror fixtures and full verification evidence.