v0.5.0

• snapshot --against <lockfile>: compare the current composition against a
  saved snapshot — bundle version/patch drift, patch-layer content changes,
  package add/remove/version, composed-tree hash. Exits 1 on drift.
• Repository-source attribution: .dsh-plugin directories under the harness
  home are collected as a fourth layer kind (repository), so rows they
  insert attribute to the repository plugin that owns them.
• Validated end-to-end with a live model: an agent called xray_composition
  in a real conversation and reported the composition summary.