fix(core): facts alter recipe NULLs embeddings before cross-dimension alters

Adversarial review: buildFactsAlterRecipe shipped the same defect class
this branch fixes for content_chunks 350 lines up — a cross-dimension
ALTER ... USING cast that pgvector refuses while rows hold old-width
vectors. Dimension changes now wipe first (the facts pipeline re-embeds
on next write); same-dim type swaps (halfvec <-> vector) keep the
lossless cast and PRESERVE data. Both behaviors pinned by tests.

src/core/embedding-dim-check.ts

@@ -575,11 +575,25 @@ export function buildFactsAlterRecipe(
 ): string {
   const opclass = columnType === 'halfvec' ? 'halfvec_cosine_ops' : 'vector_cosine_ops';
   const targetType = columnType === 'halfvec' ? `halfvec(${configuredDims})` : `vector(${configuredDims})`;
+  const dimsChanged = columnDims !== configuredDims;
   return [
     `-- ALTER ${columnType}(${columnDims}) → ${columnType}(${configuredDims}) on indexed column.`,
     `-- HOLD a maintenance window: this rewrites every row's embedding.`,
     `-- Coordinate with any active extract-conversation-facts backfill.`,
     `DROP INDEX IF EXISTS idx_facts_embedding_hnsw;`,
+    // Same-dim type swaps (halfvec <-> vector) cast row data losslessly and
+    // MUST keep it. Cross-dimension changes are different: pgvector refuses
+    // to cast existing vectors across dimensions ("expected N dimensions,
+    // not M") and aborts the transaction — and the old-space vectors are
+    // unusable at the new width anyway. NULL them first; the facts pipeline
+    // re-embeds on the next write.
+    ...(dimsChanged
+      ? [
+          `-- Dimension change: NULL embeddings BEFORE the alter — pgvector`,
+          `-- refuses cross-dimension casts and aborts the transaction.`,
+          `UPDATE facts SET embedding = NULL;`,
+        ]
+      : []),
     `ALTER TABLE facts ALTER COLUMN embedding TYPE ${targetType}`,
     `  USING embedding::${targetType};`,
     `CREATE INDEX idx_facts_embedding_hnsw`,

test/embedding-dim-check-facts.test.ts

@@ -142,6 +142,27 @@ describe('buildFactsAlterRecipe', () => {
     const recipe = buildFactsAlterRecipe(1536, 1280, 'halfvec');
     expect(recipe).toMatch(/DROP INDEX[\s\S]*ALTER TABLE[\s\S]*CREATE INDEX/);
   });
+
+  test('dimension change NULLs embeddings BEFORE the alter (pgvector refuses cross-dim casts)', () => {
+    // Same defect class fixed for content_chunks in embeddingMismatchMessage:
+    // pgvector aborts a cross-dimension ALTER while rows still hold old-width
+    // vectors. The dims-change recipe must wipe first; order pinned.
+    const recipe = buildFactsAlterRecipe(1536, 1280, 'halfvec');
+    const nullIdx = recipe.indexOf('UPDATE facts SET embedding = NULL');
+    const alterIdx = recipe.indexOf('ALTER TABLE facts ALTER COLUMN embedding TYPE');
+    expect(nullIdx).toBeGreaterThan(-1);
+    expect(alterIdx).toBeGreaterThan(-1);
+    expect(nullIdx).toBeLessThan(alterIdx);
+  });
+
+  test('same-dim type swap PRESERVES embeddings (no NULL wipe)', () => {
+    // halfvec(1536) <-> vector(1536): the USING cast is lossless and the
+    // whole point is keeping the data. A wipe here would destroy valid
+    // embeddings for no reason.
+    const recipe = buildFactsAlterRecipe(1536, 1536, 'vector');
+    expect(recipe).not.toContain('UPDATE facts SET embedding = NULL');
+    expect(recipe).toContain('USING embedding::vector(1536)');
+  });
 });
 
 describe('FactsEmbeddingDimMismatchError', () => {