fix: stop warning about merged themes in curriculum schedule

When the KB has more than 26 themes the curriculum AI is required to
merge — so theme labels not appearing as week names are expected, not
warnings. The previous validation surfaced this as a console.warn that
read like an error in the learning station console.

- validateSchedule now only flags missing theme labels when themes_kb <= 26
- adds a real coverage check: warns when learning topics are absent from
  every week (the actual signal we care about)
- adds vitest coverage for both behaviours

Closes #12

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/curriculumService.js

@@ -60,8 +60,16 @@ export function buildThemeTopicMap(topics) {
 
 /**
  * Validates a 26-week schedule against the provided topics.
- * Checks for exactly 26 weeks, duration range, theme existence, and topic existence.
- * Returns { valid: boolean, errors: string[] }
+ *
+ * Warning policy:
+ * - Theme names not appearing as a week label are NOT a warning when the KB
+ *   has more than 26 themes — the AI is required to merge in that case, and
+ *   topic_ids from the merged themes are carried through under the chosen
+ *   week label. We only warn about missing themes when merging wasn't needed.
+ * - Real coverage is measured on TOPICS: a topic that exists in the KB but
+ *   is absent from every week's topic_ids is a genuine gap and gets a warning.
+ *
+ * Returns { valid: boolean, errors: string[], warnings: string[] }
  */
 export function validateSchedule(schedule, topics) {
   const errors = [];   // Hard errors — schedule is unusable
@@ -71,10 +79,13 @@ export function validateSchedule(schedule, topics) {
     errors.push(`Schedule must contain exactly 26 weeks. Found ${schedule?.length || 0}.`);
   }
 
-  const validThemes = new Set(topics.filter(t => t.type !== 'fact' && t.learning_relevance !== 'exclude').map(t => t.theme || 'General'));
+  const learningTopics = topics.filter(t => t.type !== 'fact' && t.learning_relevance !== 'exclude');
+  const validThemes = new Set(learningTopics.map(t => t.theme || 'General'));
   const validTopicIds = new Set(topics.map(t => t.id));
+  const learningTopicIds = new Set(learningTopics.map(t => t.id));
 
   const scheduledThemes = new Set();
+  const scheduledTopicIds = new Set();
 
   for (let i = 0; i < (schedule || []).length; i++) {
     const week = schedule[i];
@@ -86,7 +97,7 @@ export function validateSchedule(schedule, topics) {
     }
     // Allow AI-merged theme names — only flag truly unknown themes as warnings
     scheduledThemes.add(week.theme);
-    
+
     if (!week.topic_ids || week.topic_ids.length === 0) {
       errors.push(`Week ${week.week_number} has no topic_ids.`);
     } else {
@@ -94,20 +105,30 @@ export function validateSchedule(schedule, topics) {
         if (!validTopicIds.has(tId)) {
           errors.push(`Week ${week.week_number} references unknown topic_id: ${tId}`);
         }
+        scheduledTopicIds.add(tId);
       }
     }
   }
 
-  // Theme coverage — soft warnings, not hard errors
-  // When there are more themes than 26 weeks, the AI must merge some.
-  const missingThemes = [];
-  for (const t of validThemes) {
-    if (!scheduledThemes.has(t)) {
-      missingThemes.push(t);
+  // Theme coverage — only warn when merging wasn't required. When themes_kb > 26
+  // the AI is *required* to merge, so absent theme labels are expected.
+  if (validThemes.size <= 26) {
+    const missingThemes = [];
+    for (const t of validThemes) {
+      if (!scheduledThemes.has(t)) {
+        missingThemes.push(t);
+      }
+    }
+    if (missingThemes.length > 0) {
+      warnings.push(`${missingThemes.length} theme(s) not scheduled: ${missingThemes.join(', ')}`);
     }
   }
-  if (missingThemes.length > 0) {
-    warnings.push(`${missingThemes.length} theme(s) not directly scheduled (topics may appear under merged themes): ${missingThemes.join(', ')}`);
+
+  // Topic coverage — the real signal. Topics carried under a merged theme are
+  // still covered; topics absent from every week are not.
+  const missingTopicCount = [...learningTopicIds].filter(id => !scheduledTopicIds.has(id)).length;
+  if (missingTopicCount > 0) {
+    warnings.push(`${missingTopicCount} learning topic(s) not covered by any week of the schedule.`);
   }
 
   return { valid: errors.length === 0, errors, warnings };
@@ -224,7 +245,9 @@ Rules:
     throw new Error(`Generated schedule failed validation after retry:\n- ${validationResult.errors.join('\n- ')}`);
   }
 
-  // Log warnings but don't fail
+  // Log warnings but don't fail. With themes_kb > 26 the AI must merge themes,
+  // so most "missing theme" noise is filtered upstream in validateSchedule —
+  // anything that lands here is a genuine coverage gap worth surfacing.
   if (validationResult.warnings.length > 0) {
     console.warn('[Curriculum] Schedule generated with warnings:', validationResult.warnings);
   }