This PR introduces a native C parser and bytecode compiler for Liquid
control flow tags, providing significant performance improvements for
template parsing and rendering.

## Performance Improvements

- Parsing: 2-10x faster depending on template complexity
- Control flow rendering: ~2x faster for if/case statements
- Variable-heavy templates: up to 10x faster parsing

## Architecture

### New Components

- **Arena Allocator** (arena.c/h): Fast bump-pointer allocation for AST nodes
- **AST Structures** (ast.c/h): 21 node types for all Liquid constructs
- **Template Parser** (template_parser.c/h): Recursive descent parser
- **Code Generator** (codegen.c/h): AST to bytecode compiler

### New VM Opcodes

Control flow:
- OP_JUMP, OP_JUMP_IF_FALSE, OP_JUMP_IF_TRUE (with wide variants)

Comparisons:
- OP_CMP_EQ, OP_CMP_NE, OP_CMP_LT, OP_CMP_GT, OP_CMP_LE, OP_CMP_GE
- OP_CMP_CONTAINS

Variables:
- OP_ASSIGN, OP_INCREMENT, OP_DECREMENT

Loop control:
- OP_FOR_INIT, OP_FOR_NEXT, OP_FOR_CLEANUP
- OP_BREAK, OP_CONTINUE

### What's Native vs Ruby

**Native C execution:**
- if/elsif/else/unless - native jump opcodes
- case/when/else - native comparison + jumps
- assign - native OP_ASSIGN
- increment/decrement - native opcodes
- All comparison operators

**Still uses Ruby (by design):**
- Custom tags (extensibility)
- Filters (Ruby filter methods)
- render/include tags
- Drops and complex objects

## Bug Fixes

- Fixed blank keyword comparison (x == blank)
- Fixed empty keyword comparison (x == empty)
- Fixed nil contains check (array contains nil)
- Fixed Bus Error crash in case/when with multiple branches
- Fixed break/continue in for loops

## Tests

- 272 unit tests passing
- 187 new tests for parser/VM functionality
- 91.5% liquid-spec conformance (108/118)