Fix object alignment for double-word boundaries (Issue #57) (#203)

* Implement proper object alignment for double-word boundaries

This commit implements Issue #57: Object alignment fixes by updating
the memory allocator to ensure all objects are aligned to 16-byte
boundaries for improved performance on modern architectures.

Key changes:
- Updated alloc_size_of() to align total allocation size to 16 bytes
- Since AllocHeader is exactly 16 bytes, objects are naturally aligned
- Added comprehensive alignment test coverage
- Fixed fragmentation test expectations for alignment impact
- Updated .gitignore to properly exclude GC test results

Performance impact: ~8% regression in allocation benchmarks due to
increased memory usage, but provides better cache alignment and
prepares for future SIMD optimizations.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix rustfmt formatting issues

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: gmorpheme

.gitignore

@@ -2,6 +2,7 @@
 **/*.rs.bk
 /.idea
 /harness/test/.result/
+/harness/test/**/.result/
 /src/syntax/grammar.rs
 /src/syntax/string_pattern.rs