fix(embed): thread write column into sumStaleChunkChars — sync cost gate followed the legacy predicate (#1262)

Review follow-up: the PR threaded the write-side column through
countStaleChunks/listStaleChunks but not sumStaleChunkChars, so the
sync cost gate counted an alt-column brain's fully-embedded corpus as
phantom backlog on every gate (inflating the --full estimate and the
deferred-mode backlog note). Both engines already share
buildStaleChunkWhere, so this is a type widening + one call-site
thread + a contrast assertion.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/commands/sync.ts

@@ -576,9 +576,16 @@ async function runInlineCostGate(
 
   // Stale backlog: cheap single SQL; fail-open to 0 so a transient DB hiccup
   // never blocks the sync. Signature-aware (model/dims swap surfaces here).
+  // #1262: follow the write-side embedding column — otherwise an alt-column
+  // brain's fully-embedded corpus counts as phantom backlog on every gate.
   let staleChars = 0;
   try {
-    staleChars = await engine.sumStaleChunkChars({ signature: currentEmbeddingSignature() });
+    const { resolveWriteColumnForEngine } = await import('../core/search/embedding-column.ts');
+    const embeddingColumn = await resolveWriteColumnForEngine(engine);
+    staleChars = await engine.sumStaleChunkChars({
+      signature: currentEmbeddingSignature(),
+      ...(embeddingColumn && { embeddingColumn }),
+    });
   } catch {
     staleChars = 0;
   }

src/core/engine.ts

@@ -1031,8 +1031,13 @@ export interface BrainEngine {
    * model signature (a model/dims swap). NULL signature is GRANDFATHERED
    * (never counted) so the post-migration corpus isn't flagged en masse.
    * Omit `signature` for the legacy `embedding IS NULL`-only count.
+   *
+   * `opts.embeddingColumn` switches the staleness predicate to the resolved
+   * write-side column (#1262) — same contract as countStaleChunks — so the
+   * sync cost gate doesn't count an alt-column brain's fully-embedded corpus
+   * as phantom backlog.
    */
-  sumStaleChunkChars(opts?: { sourceId?: string; signature?: string }): Promise<number>;
+  sumStaleChunkChars(opts?: { sourceId?: string; signature?: string; embeddingColumn?: ResolvedColumn }): Promise<number>;
   /**
    * Stamp `pages.embedding_signature = signature` for one page. Called after
    * a page's chunks are (re)embedded so a later model swap can detect it as

src/core/pglite-engine.ts

@@ -2424,7 +2424,7 @@ export class PGLiteEngine implements BrainEngine {
     return Number(count);
   }
 
-  async sumStaleChunkChars(opts?: { sourceId?: string; signature?: string }): Promise<number> {
+  async sumStaleChunkChars(opts?: { sourceId?: string; signature?: string; embeddingColumn?: ResolvedColumn }): Promise<number> {
     // Sibling of countStaleChunks: same stale predicate, summing chunk_text
     // length for the sync cost preview. ::bigint guards int4 overflow.
     const { where, params } = this.buildStaleChunkWhere(opts);

src/core/postgres-engine.ts

@@ -2579,7 +2579,7 @@ export class PostgresEngine implements BrainEngine {
     });
   }
 
-  async sumStaleChunkChars(opts?: { sourceId?: string; signature?: string }): Promise<number> {
+  async sumStaleChunkChars(opts?: { sourceId?: string; signature?: string; embeddingColumn?: ResolvedColumn }): Promise<number> {
     // Sibling of countStaleChunks: same stale predicate, summing chunk_text
     // length for the sync cost preview. ::bigint guards int4 overflow.
     const { where, params } = this.buildStaleChunkWhere(opts);

test/e2e/embedding-column-pglite.test.ts

@@ -355,6 +355,9 @@ describe('PGLite: embed --stale converges on an alt-column brain (#1262)', () =>
       // the alt-column predicate does not.
       expect(await local.countStaleChunks()).toBe(1);
       expect(await local.countStaleChunks({ embeddingColumn: descriptor })).toBe(0);
+      // sumStaleChunkChars feeds the sync cost gate — same predicate contract.
+      expect(await local.sumStaleChunkChars()).toBeGreaterThan(0);
+      expect(await local.sumStaleChunkChars({ embeddingColumn: descriptor })).toBe(0);
       expect(await local.listStaleChunks({ embeddingColumn: descriptor, batchSize: 100 })).toHaveLength(0);
       expect(await local.listStaleChunks({ batchSize: 100 })).toHaveLength(1);