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<issue_title>Constraints & Foreign Keys - SQL-99 Constraint Support</issue_title>
<issue_description>Priority: 2 (High)
Impact: SQL-99 Compliance
Test Files:test/test324.js,test/test325.js,test/test326.js,test/test327.js
Test Count: 39 tests

Problem Description

Multiple test files contain skipped tests for SQL-99 constraint support including PRIMARY KEY, FOREIGN KEY, CHECK, UNIQUE, NOT NULL, and IDENTITY constraints. These are fundamental SQL features that ensure data integrity and are essential for SQL-99 compliance.

Specific Test Cases

test324.js - Basic Constraints (13 tests)

• Lines 41-156: CREATE TABLE with constraints, INSERT violations, UPDATE violations
• Tests PRIMARY KEY, FOREIGN KEY, CHECK, NOT NULL constraints
• Tests constraint violations during INSERT and UPDATE operations

test325.js - Advanced Constraints (11 tests)

• Lines 14-143: Multiple constraint types, IDENTITY columns, DEFAULT values
• Tests IDENTITY auto-increment, DEFAULT constraints, CHECK constraints
• Tests UNIQUE constraints and complex constraint combinations

test326.js - FOREIGN KEY Relationships (9 tests)

• Lines 14-127: Foreign key references between tables
• Tests referential integrity, foreign key violations, cascading operations

test327.js - Complex Foreign Keys (8 tests)

• Lines 14-182: Self-referencing foreign keys, composite foreign keys
• Tests complex relationships, constraint naming, multiple foreign keys

Expected Behavior

AlaSQL should support SQL-99 standard constraints:

1. PRIMARY KEY: Unique identifier for table rows
2. FOREIGN KEY: Referential integrity between tables
3. CHECK: Data validation constraints
4. UNIQUE: Unique value constraints
5. NOT NULL: Mandatory field constraints
6. IDENTITY: Auto-incrementing columns
7. DEFAULT: Default value constraints

Current Status

• Test Status: All skipped (it.skip)
• Error: Unknown (tests not executed)
• Root Cause: Constraint system not implemented

Implementation Requirements

1. Parser Support

Add constraint syntax tosrc/alasqlparser.jison:

CREATETABLEtable (    column_name data_typePRIMARY KEY,    column_name data_typeNOT NULL,    column_name data_type UNIQUE,    column_name data_type DEFAULT value,    column_name data_typeCHECK (condition),    column_name data_type IDENTITY,FOREIGN KEY (column)REFERENCES other_table(column))

2. Constraint Storage

• Store constraint definitions in table metadata
• Support named and unnamed constraints
• Handle constraint inheritance and relationships

3. Constraint Enforcement

• Validate constraints during INSERT operations
• Validate constraints during UPDATE operations
• Handle constraint violation errors appropriately
• Support deferred vs immediate constraint checking

4. Foreign Key Features

• Referential integrity validation
• Cascading operations (ON DELETE/ON UPDATE)
• Self-referencing foreign keys
• Composite foreign keys

5. Advanced Features

• IDENTITY column auto-generation
• DEFAULT value evaluation
• CHECK condition evaluation
• UNIQUE constraint enforcement

SQL-99 Features Involved

• PRIMARY KEY constraints
• FOREIGN KEY constraints with REFERENCES
• CHECK constraints
• UNIQUE constraints
• NOT NULL constraints
• IDENTITY (auto-increment) columns
• DEFAULT constraints
• Constraint naming
• Composite constraints

Dependencies

• AlaSQL parser (src/alasqlparser.jison)
• Table definition system
• Data validation engine
• Error handling system
• Transaction support (for constraint rollback)

Acceptance Criteria

Basic Constraints (test324.js)

• All 13 tests enabled and passing
• PRIMARY KEY constraint enforcement
• FOREIGN KEY referential integrity
• CHECK constraint validation
• NOT NULL constraint enforcement
• Proper error messages for violations

Advanced Constraints (test325.js)

• All 11 tests enabled and passing
• IDENTITY column auto-generation
• DEFAULT value assignment
• Complex constraint combinations
• UNIQUE constraint enforcement
• Constraint naming support

Foreign Key Relationships (test326.js)

• All 9 tests enabled and passing
• Basic foreign key references
• Referential integrity validation
• Foreign key violation detection
• Cross-table relationship support

Complex Foreign Keys (test327.js)

• All 8 tests enabled and passing
• Self-referencing foreign keys
• Composite foreign keys
• Multiple foreign keys per table
• Complex relationship validation

Implementation Notes

Constraints should be:

• Enforced at data modification time
• Support both column-level and table-level definitions
• Provide clear error messages for violations
• Compatible with existing AlaSQL features
• SQL-99 compliant in syntax and behavior

Test Implementation Strate...