feat(embed): docs_embedded tracking, buffer reuse, retry hardening

Embedding pipeline improvements building on the concurrent batching
foundation:

- Track docs_embedded vs chunks_embedded separately. A document counts
  as embedded only when ALL its chunks succeed, giving accurate
  progress reporting. The sync command reads docs_embedded for its
  document count.

- Reuse a single Vec<u8> buffer (embed_buf) across all store_embedding
  calls instead of allocating per chunk. Eliminates ~3KB allocation per
  768-dim embedding.

- Detect and record errors when Ollama silently returns fewer
  embeddings than inputs (batch mismatch). Previously these dropped
  chunks were invisible.

- Improve retry error messages: distinguish "retry returned unexpected
  result" (wrong dims/count) from "retry request failed" (network
  error) instead of generic "chunk too large" message.

- Convert all hot-path SQL from conn.execute() to prepare_cached() for
  statement cache reuse (clear_document_embeddings, store_embedding,
  record_embedding_error).

- Record embedding_metadata errors for empty documents so they don't
  appear as perpetually pending on subsequent runs.

- Accept concurrency parameter (configurable via config.embedding.concurrency)
  instead of hardcoded EMBED_CONCURRENCY=2.

- Add schema version pre-flight check in embed command to fail fast
  with actionable error instead of cryptic SQL errors.

- Fix --retry-failed to use DELETE instead of UPDATE. UPDATE clears
  last_error but the row still matches config params in the LEFT JOIN,
  making the doc permanently invisible to find_pending_documents.
  DELETE removes the row entirely so the LEFT JOIN returns NULL.
  Regression test added (old_update_approach_leaves_doc_invisible).

- Add chunking forward-progress guard: after floor_char_boundary()
  rounds backward, ensure start advances by at least one full
  character to prevent infinite loops on multi-byte sequences
  (box-drawing chars, smart quotes). Test cases cover the exact
  patterns that caused production hangs on document 18526.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/commands/sync.rs

@@ -241,7 +241,7 @@ pub async fn run_sync(
             });
             match run_embed(config, options.full, false, Some(embed_cb), signal).await {
                 Ok(embed_result) => {
-                    result.documents_embedded = embed_result.embedded;
+                    result.documents_embedded = embed_result.docs_embedded;
                     embed_bar.finish_and_clear();
                     spinner.finish_and_clear();
                 }