shape-gc: Zero-pause hardware-assisted garbage collector for the Shape VM.
Replaces Arc refcounting with a concurrent mark-relocate GC using hardware pointer masking (ARM TBI / x86-64 LAM) for zero-pause collection.
Architecture
- Bump allocation with thread-local allocation buffers (TLABs) for ~1 cycle alloc
- Tri-color marking (white/gray/black) with incremental mark steps
- Hardware pointer masking to store GC metadata in upper pointer bits
- Concurrent relocation with forwarding table + SIGSEGV trap handler
- Generational collection with card table write barriers