v0.4.0 — DAG Store Garbage Collection

v0.4.0 — DAG Store Garbage Collection

Features

• DAG store garbage collection (pullrun gc, pullrun gc --apply).
  Mark-and-sweep GC with BFS reachability walk, 90% safety guard, op-lock
  protection, and CLI integration.
• Crash-atomic writes (write-then-rename with parent fsync).
• Operation locks protect in-flight pull/build/commit/import from GC.
• kernel_image_digest pinning — stopped VMs' kernel layers preserved.
• walk_reachable factored into pullrun-store and shared by
  dag_export.rs and the GC crate.

Fixes

• enumerate_gc_roots data-loss bug fixed. Changed try_read() to
  blocking read().await to guarantee root set completeness.
• BFS aborts on corrupted nodes (prevents silent subtree deletion).
• 90% safety guard measures unreachable bytes, not node count.

Tests

• 25 new tests across PRs 1–5 (134 → 159 total).