docs: update adapter specs with CURRENT_TIMESTAMP and interval_expr

- MySQL adapter: use CURRENT_TIMESTAMP (not NOW) for consistency
- Both adapters: document interval_expr() output format
  - MySQL: INTERVAL 5 SECOND
  - PostgreSQL: INTERVAL '5 seconds'

Author: claude

docs/design/multi-backend-plan.md

@@ -430,7 +430,8 @@ Key behaviors:
 - `quote_identifier()`: Returns `` `name` `` (backticks)
 - `create_table_sql()`: Includes `ENGINE=InnoDB` and inline `COMMENT`
 - `insert_sql()`: Supports `REPLACE INTO` and `ON DUPLICATE KEY UPDATE`
-- `current_timestamp_expr()`: Returns `NOW()` or `NOW(precision)`
+- `current_timestamp_expr()`: Returns `CURRENT_TIMESTAMP` or `CURRENT_TIMESTAMP(precision)`
+- `interval_expr(5, 'second')`: Returns `INTERVAL 5 SECOND`
 
 ### 2.3 PostgreSQL Adapter Implementation
 
@@ -458,7 +459,8 @@ Key behaviors:
 - `quote_identifier()`: Returns `"name"` (double quotes)
 - `create_table_sql()`: No engine; comments via separate `COMMENT ON` statement
 - `insert_sql()`: Uses `ON CONFLICT DO NOTHING` / `ON CONFLICT DO UPDATE`
-- `current_timestamp_expr()`: Returns `CURRENT_TIMESTAMP`
+- `current_timestamp_expr()`: Returns `CURRENT_TIMESTAMP` or `CURRENT_TIMESTAMP(precision)`
+- `interval_expr(5, 'second')`: Returns `INTERVAL '5 seconds'`
 - `core_type_to_sql("datetime")`: Returns `timestamp`
 - `core_type_to_sql("decimal(p,s)")`: Returns `numeric(p,s)`