Fail closed when timing.json is corrupt at compile and validate

[jmjava]

Problem

Wizard timestamps already refuse to overwrite a corrupt timing.json. Scene-compile and validate still swallowed decode errors and treated the file as missing words. Paced compile then said “run timestamps”; pace: none specs compiled as if there were no timing file.

Change

load_bundle_timing is the single loader:

• missing file → {}
• garbage JSON or a non-object root → TimestampError / SceneGenerationError

Used by scene-compile, validate (timing_sync / story_end / av_sync), scene_asset_validate, and wizard timestamps.

Milestone: milestones/timing-json-parse.md

Out of scope

• Empty segments.all still leaves timing.json unchanged (Fail-closed timestamps, concat, and Anthropic base_url #78)
• Missing timing.json still means empty words (paced compile already fails)

Testing

Local:

• ruff check src/ tests/ — clean
• pytest tests/ — 566 passed, 1 skipped
• docgen benchmark — meets src/docgen/benchmark_data/baseline.json (no clock change)

CI on this branch is the merge gate.