fix(notion-fetch): correct skip reason logic to detect impossible conditions

Fix logic bug in skip reason detection where unreachable branches
were incorrectly labeled as "should not skip" instead of being
identified as logic errors.

Problem:
- Inside `if (!needsProcessing)` block, we know ALL conditions
  for needsProcessing are FALSE due to boolean OR logic
- Previous labels like "should not skip" were misleading since
  those branches are mathematically unreachable
- Would never detect actual logic bugs in needsProcessing calculation

Solution:
- Change impossible condition messages to "🔴 ERROR:" prefix
- Add clear comments explaining unreachability
- Document that ONLY "unchanged since [timestamp]" is valid
- If ERROR appears in logs, indicates bug in needsProcessing logic

Benefits:
- Proper detection of logic bugs (if they exist)
- Clear distinction between valid skip (cache hit) vs impossible cases
- Better debugging when investigating Issue #95

Related: ISSUE_95_PLAN.md Phase 1 review

Author: claude

scripts/notion-fetch/generateBlocks.ts

@@ -712,23 +712,35 @@ export async function generateBlocks(
 
           if (!needsProcessing) {
             // Page unchanged, skip processing but still count it
-            // Determine the specific reason for skipping (for diagnostics)
+            // Since !needsProcessing is true, we know ALL these conditions are false:
+            //   - syncMode.fullRebuild === false
+            //   - cachedPage exists
+            //   - no missing outputs
+            //   - path hasn't changed
+            //   - timestamp hasn't changed
+            // If any "ERROR:" appears in logs, it indicates a logic bug in needsProcessing calculation
             let skipReason: string;
             if (syncMode.fullRebuild) {
-              // This condition can't be reached if !needsProcessing, but for completeness
-              skipReason = "full rebuild mode";
+              // Should be unreachable (fullRebuild would make needsProcessing=true)
+              skipReason = "🔴 ERROR: fullRebuild=true but !needsProcessing";
             } else if (!cachedPage) {
-              skipReason = "not in cache (NEW PAGE - should not skip)";
+              // Should be unreachable (!cachedPage would make needsProcessing=true)
+              skipReason = "🔴 ERROR: not in cache but !needsProcessing";
             } else if (hasMissingOutputs(metadataCache, page.id)) {
-              skipReason = "missing output files (should not skip)";
+              // Should be unreachable (missing outputs would make needsProcessing=true)
+              skipReason =
+                "🔴 ERROR: missing output files but !needsProcessing";
             } else if (!cachedPage.outputPaths?.includes(filePath)) {
-              skipReason = `path changed from [${cachedPage.outputPaths?.join(", ") || "none"}] to [${filePath}] (should not skip)`;
+              // Should be unreachable (path change would make needsProcessing=true)
+              skipReason = `🔴 ERROR: path changed [${cachedPage.outputPaths?.join(", ") || "none"}] → [${filePath}] but !needsProcessing`;
             } else {
               const notionTime = new Date(page.last_edited_time).getTime();
               const cachedTime = new Date(cachedPage.lastEdited).getTime();
               if (notionTime > cachedTime) {
-                skipReason = `timestamp updated (${page.last_edited_time} > ${cachedPage.lastEdited}) (should not skip)`;
+                // Should be unreachable (newer timestamp would make needsProcessing=true)
+                skipReason = `🔴 ERROR: timestamp newer (${page.last_edited_time} > ${cachedPage.lastEdited}) but !needsProcessing`;
               } else {
+                // This is the ONLY valid reason for !needsProcessing
                 skipReason = `unchanged since ${cachedPage.lastEdited}`;
               }
             }