Unify completion facts with execution time

[mixn1998]

What changed

• Make completion facts the sole persisted execution facts.
• Store optional actual execution segments on each completion fact.
• Show untimed completions in Timeline's top “未配置时间” area.
• Add deterministic v16 → v17 migration and regression coverage.

Why

The independent execution ledger could be empty while completion facts existed, making completed work disappear from Timeline.

Validation

• npm run typecheck
• npm test — 151 passing
• npm run build
• Current local database regression: 34/34 current-week completion facts visible