chore: harden text chunk embeddings and text content storage

Align text chunk embedding identity with knowledge entities (chunk id as record id, UNIQUE chunk_id index, dimension validation), make cascade deletes transactional, and improve text content patch/search reliability with tests.

common/migrations/20260528_000003_text_chunk_embedding_storage_hardening.surql

@@ -0,0 +1,21 @@
+-- Harden text chunk embeddings storage invariants.
+
+-- Re-key embeddings so record id matches chunk id (stable 1:1 identity).
+FOR $emb IN (SELECT * FROM text_chunk_embedding) {
+    LET $chunk_key = record::id($emb.chunk_id);
+    LET $canonical = type::thing('text_chunk_embedding', $chunk_key);
+    IF $emb.id != $canonical {
+        UPSERT $canonical CONTENT {
+            chunk_id: $emb.chunk_id,
+            embedding: $emb.embedding,
+            user_id: $emb.user_id,
+            source_id: $emb.source_id,
+            created_at: $emb.created_at,
+            updated_at: $emb.updated_at
+        };
+        DELETE $emb.id;
+    }
+};
+
+REMOVE INDEX IF EXISTS text_chunk_embedding_chunk_id_idx ON text_chunk_embedding;
+DEFINE INDEX IF NOT EXISTS text_chunk_embedding_chunk_id_idx ON text_chunk_embedding FIELDS chunk_id UNIQUE;

common/migrations/definitions/20260528_000003_text_chunk_embedding_storage_hardening.json

@@ -0,0 +1 @@
+{"schemas":"--- original\n+++ modified\n@@ -237,7 +237,7 @@\n\n -- Indexes\n -- DEFINE INDEX IF NOT EXISTS idx_embedding_text_chunk_embedding ON text_chunk_embedding FIELDS embedding HNSW DIMENSION 1536;\n-DEFINE INDEX IF NOT EXISTS text_chunk_embedding_chunk_id_idx ON text_chunk_embedding FIELDS chunk_id;\n+DEFINE INDEX IF NOT EXISTS text_chunk_embedding_chunk_id_idx ON text_chunk_embedding FIELDS chunk_id UNIQUE;\n DEFINE INDEX IF NOT EXISTS text_chunk_embedding_user_id_idx ON text_chunk_embedding FIELDS user_id;\n DEFINE INDEX IF NOT EXISTS text_chunk_embedding_source_id_idx ON text_chunk_embedding FIELDS source_id;\n\n","events":null}