Fail closed when wizard state segments is not a mapping

[jmjava]

Problem

load_state accepts any JSON object. GET /api/segments then does state.get("segments", {}).get(seg_id, {}). A list, string, or per-id non-object under segments raises AttributeError. POST /api/state writes that payload unchanged.

Corrupt JSON / a non-object root still reset to {"segments": {}} so a broken file does not brick the wizard.

Change

When segments is present, it must be a JSON object whose values are objects. Missing / null segments still means {}.

• GET /api/state and GET /api/segments return 500 with WizardError (not a traceback)
• POST /api/state returns 400 for a non-object body or invalid segments, and does not write the file

Tests

Local: ruff check src/ tests/ green; pytest tests/ 741 passed, 1 skipped; docgen benchmark meets src/docgen/benchmark_data/baseline.json (no --update-baseline).

• load_state still resets corrupt JSON
• list / scalar segments and per-id non-objects raise
• null segments becomes {}
• API GET/POST coverage above

Out of scope

• Changing the corrupt-JSON → empty reset
• Wizard except Exception around narration_topic_label