Concurrent GC (eliminate stop-the-world)

## 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.