feat: canonical SQL DDL + schema validator + migration tool

- schema/canonical.sql: 29 tables across 3 databases, CHECK constraints,
  foreign keys, 13 indexes, WAL mode, schema_version tracking
- tools/validate-schema.ts: applies DDL to in-memory SQLite, extracts
  PRAGMA table_info + sqlite_master metadata as JSON
- tools/migrate-db.ts: CLI for Tauri→Electrobun data migration with
  atomic transaction, version fencing, INSERT OR IGNORE
- docs/SWITCHING.md: migration guide with prerequisites and troubleshooting

tools/validate-schema.ts

@@ -0,0 +1,109 @@
+#!/usr/bin/env bun
+/**
+ * validate-schema.ts — Apply canonical.sql to an in-memory SQLite database
+ * and extract structural metadata for CI comparison.
+ *
+ * Usage: bun tools/validate-schema.ts
+ * Output: JSON to stdout with tables, columns, indexes, and schema version.
+ */
+
+import { Database } from "bun:sqlite";
+import { readFileSync } from "fs";
+import { join } from "path";
+
+// ── Load canonical DDL ────────────────────────────────────────────────────────
+
+const schemaPath = join(import.meta.dir, "..", "schema", "canonical.sql");
+let ddl: string;
+try {
+  ddl = readFileSync(schemaPath, "utf-8");
+} catch (err) {
+  console.error(`Failed to read ${schemaPath}: ${err}`);
+  process.exit(1);
+}
+
+// ── Apply to in-memory DB ─────────────────────────────────────────────────────
+
+const db = new Database(":memory:");
+
+try {
+  db.exec(ddl);
+} catch (err) {
+  console.error(`Schema application failed: ${err}`);
+  process.exit(1);
+}
+
+// ── Extract metadata ──────────────────────────────────────────────────────────
+
+interface ColumnInfo {
+  cid: number;
+  name: string;
+  type: string;
+  notnull: number;
+  dflt_value: string | null;
+  pk: number;
+}
+
+interface TableMeta {
+  name: string;
+  type: string; // 'table' | 'virtual'
+  columns: ColumnInfo[];
+  indexes: string[];
+}
+
+// Get all tables and virtual tables from sqlite_master
+const masterRows = db
+  .prepare(
+    `SELECT name, type FROM sqlite_master
+     WHERE type IN ('table', 'virtual table')
+       AND name NOT LIKE 'sqlite_%'
+       AND name NOT LIKE '%_content'
+       AND name NOT LIKE '%_data'
+       AND name NOT LIKE '%_idx'
+       AND name NOT LIKE '%_config'
+       AND name NOT LIKE '%_docsize'
+     ORDER BY name`,
+  )
+  .all() as Array<{ name: string; type: string }>;
+
+const tables: TableMeta[] = [];
+
+for (const { name, type } of masterRows) {
+  // Get column info (not available for FTS5 virtual tables)
+  let columns: ColumnInfo[] = [];
+  try {
+    columns = db
+      .prepare(`PRAGMA table_info('${name}')`)
+      .all() as ColumnInfo[];
+  } catch {
+    // FTS5 tables don't support table_info
+  }
+
+  // Get indexes for this table
+  const indexRows = db
+    .prepare(
+      `SELECT name FROM sqlite_master
+       WHERE type = 'index' AND tbl_name = ?
+       ORDER BY name`,
+    )
+    .all(name) as Array<{ name: string }>;
+
+  tables.push({
+    name,
+    type: type === "table" ? "table" : "virtual",
+    columns,
+    indexes: indexRows.map((r) => r.name),
+  });
+}
+
+// ── Output ────────────────────────────────────────────────────────────────────
+
+const output = {
+  schemaFile: "schema/canonical.sql",
+  version: 1,
+  tableCount: tables.length,
+  tables,
+};
+
+console.log(JSON.stringify(output, null, 2));
+db.close();