fix: rebuild incompatible birth-time candidate models

frontend/src/lib/birth-time-dynamic-engine-input.ts

@@ -1,3 +1,4 @@
+import { z } from "zod";
 import type {
   DifferencePacketInput,
   DynamicChoiceScoreInput,
@@ -6,6 +7,14 @@ import type {
 import type { ServerChoiceEvidence } from "./birth-time-dynamic-choice-internal.ts";
 import type { TimeRange } from "./birth-time-dynamic-choice.ts";
 
+const reusableCandidateModelSchema = z.object({
+  opportunity_model_version: z.literal("birth-time-opportunity-model-v2"),
+  range: z.object({ start_time: z.string(), end_time: z.string() }),
+  windows: z.array(z.object({
+    activations: z.record(z.string(), z.number().finite().nonnegative()),
+  }).passthrough()),
+}).passthrough();
+
 export class BirthTimeDynamicEngineInputError extends Error {
   readonly name = "BirthTimeDynamicEngineInputError";
 }
@@ -51,11 +60,10 @@ function candidateModelForRange(
   range: TimeRange,
 ): Readonly<Record<string, unknown>> | null {
   if (model === null) return null;
-  if (model.opportunity_model_version !== "birth-time-opportunity-model-v2") return null;
-  const persistedRange = model.range;
-  if (typeof persistedRange !== "object" || persistedRange === null) return model;
-  const value = persistedRange as Readonly<Record<string, unknown>>;
-  return value.start_time === range.startTime && value.end_time === range.endTime
+  const parsed = reusableCandidateModelSchema.safeParse(model);
+  if (!parsed.success) return null;
+  return parsed.data.range.start_time === range.startTime
+    && parsed.data.range.end_time === range.endTime
     ? model
     : null;
 }

frontend/tests/birth-time-dynamic-engine-input.test.ts

@@ -6,6 +6,15 @@ import {
 } from "../src/lib/birth-time-dynamic-engine-input.ts";
 import { dynamicCase } from "./birth-time-dynamic-persistence-fixture.ts";
 
+function matchingCandidateModel(activation = 0) {
+  return {
+    version: "birth-time-choice-scoring-v2",
+    opportunity_model_version: "birth-time-opportunity-model-v2",
+    range: { start_time: "05:02", end_time: "05:03" },
+    windows: [{ activations: { "05:02": activation, "05:03": 1 } }],
+  };
+}
+
 function narrowedCase(startTime = "05:02", endTime = "05:03") {
   const stored = dynamicCase();
   return {
@@ -65,16 +74,22 @@ test("narrowed question generation rebuilds a candidate model for the current ra
 
 test("matching candidate models remain reusable", () => {
   const stored = narrowedCase();
-  const matchingModel = {
-    version: "birth-time-choice-scoring-v2",
-    opportunity_model_version: "birth-time-opportunity-model-v2",
-    range: { start_time: "05:02", end_time: "05:03" },
-  };
+  const matchingModel = matchingCandidateModel();
   const input = dynamicDifferenceInput({ ...stored, candidateModel: matchingModel });
 
   assert.equal(input.candidateModel, matchingModel);
 });
 
+test("persisted candidate models with legacy negative activations are rebuilt", () => {
+  const stored = narrowedCase();
+  const input = dynamicDifferenceInput({
+    ...stored,
+    candidateModel: matchingCandidateModel(-0.2),
+  });
+
+  assert.equal(input.candidateModel, null);
+});
+
 test("evidence projection preserves cross-midnight candidate chronology", () => {
   const stored = narrowedCase("23:59", "00:00");
   const input = dynamicChoiceScoreInput({