feat: implement block merging optimization (Issue #17)

Implements Phase 1 of block merging: nested block consolidation.

**Key Features:**
- Merges nested blocks with compatible types
- Recursive processing of all control flow structures
- Type-safe merging with compatibility checking
- Reduces CFG complexity and improves code locality

**Implementation:**
- Pattern-based nested block detection
- Bottom-up recursive merging (innermost first)
- Type compatibility validation (Empty, Value, Func signatures)
- O(n) single-pass algorithm with recursion

**Optimizations Applied:**
1. Simple nested blocks → merged into parent
2. Triple/multiple nesting → fully flattened
3. Blocks with prefix content → merged preserving order
4. Blocks within if/loop → independently merged
5. Empty nested blocks → consolidated

**Test Coverage:**
- 5 comprehensive tests for all merging patterns
- All 32 library tests passing
- End-to-end validation with test fixtures
- All benchmarks produce valid WebAssembly

**Design Documentation:**
- Full design document in docs/BLOCK_MERGING_DESIGN.md
- Documents Phase 1 (current) implementation
- Outlines Phase 2 (CFG-based) and Phase 3 (sequential) enhancements

**Pipeline Integration:**
Runs after DCE in the optimization pipeline:
Constant Folding → Branch Simplification → DCE → Block Merging

**Performance:**
- Maintains ~61% average size reduction across benchmarks
- All 7 benchmarks produce valid WASM
- Typical 10-15% reduction in block count

Author: avrabe