[HansRobo]

Summary

Adds an on-disk cache for the package dependency graph (release R7 of the
review-and-proposals roadmap, minor / v6.4.0).

DependencyGraphAnalyzer.scan() re-parsed every package.xml with catkin_pkg
on each invocation, which is a measurable cost on autoware-scale workspaces
where ws build / inspect run the scan repeatedly. The cache lets a second
scan of an unchanged tree skip the XML parse entirely.

How it works

• Key: a sha256 fingerprint of every package.xml under src/ — each
  file contributes its relative path and the sha256 of its content, so
  edits (even same-second), additions, removals, and renames all invalidate
  the cache. mtime is intentionally not used.
• Storage: serialised as JSON under .cwm/cache/dag/<fingerprint>.json
  (the previously-defined-but-unused cache_path). Written atomically
  (tmp + os.replace) so concurrent worktrees/agents can't corrupt it.
  Package paths are stored relative to src, so worktrees with identical
  package sets share the entry.
• API: scan() gains an opt-in cache_dir kwarg and stays backward
  compatible when omitted. compute_changeset passes config.dag_cache_dir.
• Invalidation is fully automatic — no CLI flag or env var. A cache miss
  (absent / stale / corrupt / format-version bump) falls back silently to a
  full rebuild.

Changes

• new src/cwm/core/dag_cache.py — compute_fingerprint, load_cached_graph, store_cached_graph, CACHE_FORMAT_VERSION
• dependency_graph.py — shared iter_package_xmls() walk (single source of truth for _rebuild_from_src + fingerprint), scan(cache_dir=...) load-or-rebuild, to_dict/from_dict
• config.py — dag_cache_dir property
• changeset.py — pass cache_dir into scan
• tests — test_dag_cache.py (new) + TestDGACache; shared make_package_xml/write_package helpers in conftest.py

Testing

• uv run pytest -q → 228 passed
• Key coverage: cache hit skips parse_package (monkeypatched to fail),
  rebuild on edit/add/remove, format-version mismatch, corrupt-cache fallback,
  cross-worktree sharing with correct path restoration.