fix: flush surreal writer after symbol embeddings persistence

Critical bug: symbol_embeddings were being queued but not flushed before
edge resolution phase, causing symbol_embeddings table to be incomplete
or empty.

Root cause: precompute_symbol_embeddings() and
precompute_unresolved_symbol_embeddings() persist embeddings at lines
902 and 938, but no flush occurred until line 1231 (after edge resolution).

Fix: Added flush_surreal_writer() call at line 979 after symbol embedding
precomputation completes but before edge resolution begins.

This ensures symbol embeddings are persisted to SurrealDB before being
used for AI semantic matching during edge resolution.

Complete flush audit:
- ✅ nodes: flush at line 733
- ✅ edges: flush at line 1231
- ✅ project_metadata: flush at line 1231
- ✅ file_metadata: flush at line 1237 (fixed in previous commit)
- ✅ symbol_embeddings: flush at line 979 (THIS FIX)

Author: jruokola

crates/codegraph-mcp/src/indexer.rs

@@ -975,6 +975,9 @@ impl ProjectIndexer {
                 )
             };
 
+            // Flush symbol embeddings to database before edge resolution
+            self.flush_surreal_writer().await?;
+
             let mut unresolved_edges = 0;
             let mut exact_matches = 0;
             let mut pattern_matches = 0;