Show lazy-skipped nodes as 'skipped' (fix stuck-on-running)

[bscholer]

Summary

Closes a follow-up to #175. The runtime emits `node_skipped` for lazy-skipped nodes, but the UI state machine had no case for it, so the prior `node_started` left the node stuck on `status='running'` indefinitely. Now visible any time you tweak a downstream node and the upstream chain stays cached.

• Add a new `'skipped'` NodeStatus rather than reusing `'cached'`. Cached entries have files on disk that the preview pane can load; skipped entries don't (the runtime decided nothing downstream would read them). Conflating the two would make the preview pane try to load an image that isn't there.
• Add `node_skipped` to the UI `JobEventType` union and add the matching `case 'node_skipped'` to `usePipelineState`'s state machine.
• Backend `EventType` literal picks up the long-missing `node_skipped` and `node_warning` so the cast in `_emit_event` matches reality.
• Mirror `cached` styling (muted) for the badge. The label "skipped" carries the distinction.

Test plan

• `npm run check` (svelte-check): 0 errors
• Backend tests pass (test_lazy_rerun, test_jobs_persistence)
• Ruff clean
• In a browser: tweak the stretch node on a project whose register/stack are cached; confirm the upstream nodes flip to "skipped" (not stuck "running")