Fail closed when timing.json stems are not JSON objects

[jmjava]

Problem

load_bundle_timing already rejects corrupt JSON and a non-object root. Per-stem values were untyped.

A list, string, number, or null under a stem then:

1. Compile / scene-spec-generate treated the stem as missing words (pace: none compiled as if timestamps had never been run).
2. sync_audio_tail_waits_in_scenes called .get on a list and raised AttributeError.
3. Wizard freshness treated a list stem as a present timestamps entry.

Change

Every present timing.json stem value must be a JSON object. Missing file is still {}. A present non-object stem raises TimestampError (timing.json['stem'] must be a JSON object, not …).

sync_audio_tail_waits_in_scenes skips a stem only when the payload is a dict without segments.

Tests

Local: ruff check src/ tests/ green; pytest tests/ 725 passed, 1 skipped; docgen benchmark meets src/docgen/benchmark_data/baseline.json (no --update-baseline).

• load_bundle_timing / extract_all reject list and scalar stems without rewriting the file
• linted_class_block_from_spec (pace: none) fails on a list stem
• validate timing_sync, scene-asset checks, and wizard statuses surface the error
• sync_audio_tail_waits_in_scenes raises SceneGenerationError instead of AttributeError

Out of scope

• Empty segments.all still leaves timing.json unchanged (Fail-closed timestamps, concat, and Anthropic base_url #78)
• Inner words / segments list typing (paced compile already fails when words is missing)
• Bootstrap _load_timing helpers inside compiled scenes.py