docs: log Batch 02, and the habit three pieces of work have now established

Three consecutive guards and models were wrong in ways reading them would never
have shown, and each was caught by running it: the Room schema check that Room
itself defeated, the boundary check that examined nothing, and three modelling
faults in the engine. Recorded as a habit rather than three coincidences.

docs/history/DEVELOPMENT_LOG.md

@@ -32,6 +32,69 @@ written and stay true. It is exempt from review for the same reason a receipt is
 
 ## Entries
 
+### 2026-08-18 — Batch 02 shipped: the prediction engine, and three bugs the tests found
+
+All five Batch 02 issues closed and the milestone closed with them. The app now
+ships the engine PRODUCT_PLAN.md §12 specifies rather than the prototype it was
+explicitly labelled as.
+
+**The shape is the design.** `PersonalPredictionEngine` keeps a discrete
+probability distribution over candidate start dates, not a date with a margin.
+The mode is the forecast, the window is the narrowest span holding 80% of the
+mass, and a "Not yet" is the distribution conditioned on what the user said.
+§13's requirement — that a "Not yet" updates the date, the window *and* the
+confidence rather than shifting a fixed prediction by a day — is not extra work
+in that design; it is the only thing that structure can do.
+
+**It is better, as a number.** `EngineComparisonTest` scores both engines over
+the §51 fixtures on every build: mean absolute error 0.67 against 1.00, and the
+window contained the actual start 9 times out of 9 against 7.
+
+**Coverage is the measure, not width — and measuring taught that.** The first
+version of the comparison asserted the new windows must not be wider. It failed,
+and it was the assertion that was wrong: where the personal engine is wider, it
+is right to be, and the baseline answers a history with a suspected missing
+period using a two-day window and misses. A window that misses is a broken
+promise rather than a tight forecast.
+
+**Three modelling bugs, each found by a test failing rather than by reading:**
+
+1. Median absolute deviation alone reads a user alternating 25 and 37 days as
+   perfectly consistent — half her deviations are zero. Twenty disagreeing
+   cycles came back High, which is exactly the §15 rule about volume not buying
+   confidence. Spread is now the larger of MAD and mean absolute deviation.
+2. Recency weighting assumes the recent past predicts the near future. For a
+   variable user that is false, and weighting it equally cost three days on the
+   §51 variable fixture. Recency is now trusted in proportion to how much her
+   cycles agree — which is a better statement of what recency weighting is
+   actually for.
+3. A fixed one-day floor on trend detection fired on a 42-day-cycle history
+   whose medians differed by one day. One day is a real trend at 28 and rounding
+   error at 42, so the floor is relative to the user's own spread.
+
+That is now three consecutive pieces of work — the schema guard, the boundary
+guard, and the engine — where the thing that found the defect was **running it,
+not reading it**. Worth stating as a habit rather than a coincidence.
+
+**Wired through, not just tested.** `PredictionInput` carries recent absolute
+errors and the repository feeds scored errors back in. Without that the app
+would store every error it makes and never read one back — measuring accuracy
+rather than learning from it, with §12 step 5's widening happening only in a
+unit test.
+
+- **Closed:** #10, #11, #12, #13, #14 — and the `Batch 02 — Prediction Engine`
+  milestone, which closing the last issue does not do.
+- **Next action:** Batch 03 — Core UX. The onboarding flow in §19, the designed
+  Today screen and its six dynamic states in §21–§22, period start and end
+  logging in §23–§24, the calendar in §26 and Insights in §27. The working
+  surface built in Batch 01 is deliberately ugly and says so on screen; it is
+  the thing Batch 03 replaces. Before starting, file the Batch 03 issues — the
+  milestone exists and is empty.
+- **Blockers:** Unchanged and both need a person. #8, the three branding marks,
+  which an agent must not fake. #9, the webhook, whose URL and secret are not
+  readable from this machine — until it is registered an opened `P0` raises no
+  alert at all. Neither blocks Batch 03.
+
 ### 2026-08-18 — Batch 01 foundation: seven of nine issues, and three guards that were wrong
 
 Room, DataStore, the repository layer, period CRUD end to end on a device, and