Prototype: declarative PostWriteReconcilePlan

[kyleve]

Prototype (draft) — stacked on #184

Exploratory follow-on to TLA+ protocol expansion for Where. Not merge-ready — validates whether a declarative post-write fan-out plan makes PostWriteReconcile easier to digest and extend.

Hypothesis

Post-write behavior is a fixed pipeline with a policy fork (sample ingest vs day-data change vs issue-only). That fork should be a pure plan builder; DayJournal should only execute steps.

What this adds

• PostWriteReconcilePlan — PostWriteOutcome, ReconcileStep, and forOutcome(_:)
• DayJournal.execute(_:) — sequential step runner
• PostWriteReconcilePlanTests — asserts plan shape without WidgetKit mocks

Non-goals

• Routing DailySummaryReconciler or setPrimaryRegions (still in TODOs)
• Changing fan-out ordering or widget skip policy
• Merging with the tracking pure-core prototype (Prototype: declarative TrackingReconcile pure core #188) — independent draft

Verification

• PostWriteReconcilePlanTests
• DayJournalTests (full suite on prototype branch)

Next step if this lands

Adding DailySummaryReconciler becomes extending forOutcome / adding a step — not touching every write call site.