perf(db): drop redundant idx_edges_source / idx_edges_target (#142)

Both narrow indexes are fully covered by the existing (source, kind)
and (target, kind) composites via SQLite's left-prefix scan, so
they're dead weight on every write. Empirical measurements (from the
spike script in PR #122 on a 50K-node / 250K-edge synthetic DB):

  - DB size: 34.7 MB → 27.0 MB (-22.2%)
  - Bulk insert (250K edges): 590ms → 431ms (1.37× faster)
  - source/target lookup latency: no regression

Adds migration v4 to drop both on existing databases; fresh-DB schema
no longer creates them.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

__tests__/foundation.test.ts

@@ -305,7 +305,7 @@ describe('Database Connection', () => {
 
     const version = db.getSchemaVersion();
     expect(version).not.toBeNull();
-    expect(version?.version).toBe(3);
+    expect(version?.version).toBe(4);
 
     db.close();
   });

__tests__/pr19-improvements.test.ts

@@ -299,7 +299,7 @@ describe('Best-Candidate Resolution', () => {
 describe('Schema v2 Migration', () => {
   it.skipIf(!HAS_SQLITE)('should have correct current schema version', async () => {
     const { CURRENT_SCHEMA_VERSION } = await import('../src/db/migrations');
-    expect(CURRENT_SCHEMA_VERSION).toBe(3);
+    expect(CURRENT_SCHEMA_VERSION).toBe(4);
   });
 
   it.skipIf(!HAS_SQLITE)('should have migration for version 2', async () => {