feat(ddl): TRUNCATE TABLE typed builder with full PG grammar

[productdevbook]

Summary

• Adds a first-class truncate(table | tables) factory + db.schema.truncate(...) that surfaces the full PostgreSQL TRUNCATE grammar (ONLY, RESTART IDENTITY / CONTINUE IDENTITY, CASCADE / RESTRICT, comma-separated multi-table list).
• Refuses cleanly on dialects without the grammar: MySQL / MSSQL accept only the simple TRUNCATE TABLE <name> form (no multi-table, no modifiers — printer points at ALTER TABLE … AUTO_INCREMENT / DBCC CHECKIDENT for the identity-reset case), SQLite refuses outright with a pointer at db.deleteFrom(t).allRows() as the workaround.
• Keeps the existing single-table db.schema.truncateTable(name) + TruncateTableBuilder working unchanged so no caller breaks.
• New feature flags TRUNCATE_TABLE (PG/MySQL/MSSQL) and TRUNCATE_PG_OPTIONS (PG only). Recipe section added to docs/recipes.md.

Test plan

• pnpm fmt && pnpm lint && pnpm typecheck all green
• pnpm vitest run — 2507 tests pass (41 new in test/ddl/truncate.test.ts)
• PG simple / multi-table / ONLY / RESTART IDENTITY / CASCADE / all-in-combination emission
• MySQL + MSSQL refuse every PG-specific modifier
• SQLite refuses outright with pointer at DELETE FROM
• PGlite roundtrip: insert rows + TRUNCATE → count = 0
• PGlite roundtrip: TRUNCATE … RESTART IDENTITY resets sequence
• PGlite roundtrip: multi-table TRUNCATE empties both
• Back-compat: existing db.schema.truncateTable(name) tests still pass

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