Fail closed when Grok STT coerces bool start to 1.0

[jmjava]

Problem

load_bundle_timing already requires JSON-number start / end (#134). Grok STT ingest still did float(w.get("start") or 0.0).

start: true became 1.0. Missing / null start became 0.0 and dumps the board. duration: true became 1.0. That payload is what timestamps --engine whisper writes into timing.json when ai.provider is grok.

Change

• _stt_json_number requires int/float (not bool) for word/segment start / end and present duration
• Missing duration still falls back to last word end
• Explicit start: 0.0 stays 0

Gate

• ruff check src/ tests/ green
• pytest tests/ — 812 passed, 1 skipped
• docgen benchmark meets src/docgen/benchmark_data/baseline.json (no clock change; no --update-baseline)

Out of scope

• OpenAI whisper-1 SDK path (typed .start / .end)
• Requiring end >= start

Milestone: milestones/grok-stt-start-end.md