feat: Support SELECT FOR UPDATE/SHARE (row locking)

[ajitpratap0]

Description

Add support for row-level locking clauses in SELECT statements.

Example SQL

-- Basic FOR UPDATE
SELECT * FROM accounts WHERE id = 1 FOR UPDATE;

-- FOR SHARE (shared lock)
SELECT * FROM products WHERE category = 'electronics' FOR SHARE;

-- With NOWAIT
SELECT * FROM inventory WHERE sku = 'ABC' FOR UPDATE NOWAIT;

-- With SKIP LOCKED
SELECT * FROM jobs WHERE status = 'pending' FOR UPDATE SKIP LOCKED;

-- Specific tables
SELECT * FROM orders o JOIN items i ON o.id = i.order_id FOR UPDATE OF orders;

Current Behavior

FOR UPDATE/SHARE clauses are not parsed.

Expected Behavior

Parser should recognize FOR UPDATE/SHARE with optional NOWAIT, SKIP LOCKED, and OF table_name modifiers.

Priority

Medium - important for transaction safety

Implementation Notes

• Add ForClause AST node with LockType, Tables, NoWait, SkipLocked fields
• Add to SelectStatement AST structure
• Support multiple lock types: UPDATE, SHARE, NO KEY UPDATE, KEY SHARE