Concurrent GC (eliminate stop-the-world)

[timsehn]

Problem

dolt_gc() rewrites the entire chunk store file in a single atomic operation, blocking all access. Sweep phase buffers all surviving chunks in RAM.

Current behavior

• Mark: BFS from all roots, builds in-memory hash set
• Sweep: loads ALL surviving chunks into buffer, writes temp file, atomic rename
• Blocks all readers and writers during entire process

Expected behavior

• Generational GC: separate old vs new chunks
• Concurrent collection: readers continue on old snapshot while GC builds new one
• Incremental marking: don't buffer entire file in RAM

Impact

Repos >10GB can't GC within typical maintenance windows.