Graph snapshot diff (read path)

Structural comparison between two columnar v2 snapshots for PR gates.

Layers

Layer	Module	Role
L0	`snapshot_diff::SnapshotPair`	Parallel open; digest fast path
L1	`stable_key::StableNodeKey`	BLAKE3(path, name, type) — UUID-independent
L2	`snapshot_diff`	`DiffSink`, node hash index, edge merge-join on stable keys
L3	`rgctl_incremental::pr_scope`	Git `name-status` paths + hunk line overlap
L3b	`rgctl_incremental::cascade`	Reverse `Calls` expansion for caller re-extract
L4	`pr-check` CLI	Temporal policy on scoped head entities
L5	`policy_diff`	NEW / EXISTING / RESOLVED classification
L6	`check --base-ref/--head-ref/--strict`	Working-tree / PR diff scoping fixes

Hot-path rules

Mmap-only string pool reads via string_at (no String alloc on keys).
Edge diff: map UUID → StableNodeKey, sort stable edge keys, merge-join (not edge_meta: Vec collect).
Node index on head: rayon shard by idx % 16, merge disjoint maps.
Sync CPU; async hosts use spawn_blocking at the boundary only.

Benchmarks

cargo bench -p rgctl-graph --bench snapshot_diff

Groups: digest_fast_path_equal, node_index_parallel, edge_merge_join, full_diff_noop_sink.

Reverse-dependency cascade

When a callee file changes (especially renames), call sites in untouched caller files must be re-parsed so Calls edges target the new stable node IDs.

ChangeSet.invalidation_paths() yields modified/deleted/rename-old paths.
incoming_callers_files_depth(base_mmap, seeds, N) scans incoming Calls edges into nodes declared in those files and returns caller declaring files.
IncrementalUpdater unions cascaded paths with extract_paths() before delta extract + rebuild_relations.

--cascade-depth (default 1, 0 = disabled) on incremental update and pr-check (used when delta head synthesis lands in Phase 4).

Scoped policy analysis (Phase 5)

pr-check no longer dual-hydrates full graphs for blast/centrality:

Resolve scoped entity UUIDs from StableNodeKey on each snapshot.
collect_upstream_call_closure → mmap Calls reverse BFS.
hydrate_subset → small MemoryBackend for policy node lookups.
BlastRadiusEngine::build_scoped on the upstream closure (parity with full graph for scoped seeds).
Centrality: reuse .rgctl/analysis_results.bin when node count matches; else analyze_scoped on seed entities only.