Harden the async node kinds: per-node runs, and failures on the node

[dovvnloading]

Problem

Three related gaps in how the plugin node kinds handle asynchronous work.

Artifact could not report a failure. ArtifactState carried exactly one field, artifact_content, so a failed generation surfaced only through a session-wide notification toast - with two artifact nodes on a canvas there was no way to tell which one failed. Web Research already keeps research_error on the node, and Gitlink and Code Sandbox render their own in-card banners.

Artifact and Web Research allowed one run at a time across the whole canvas. RunRegistry.is_busy() is kind-scoped and session-wide - RunHandle's own docstring states node_id is "informational only … NEVER consulted" - so a second research node could not start while an unrelated one was in flight, on a canvas whose premise is parallel branches, and the refusal could not name the node holding the slot. Gitlink and Code Sandbox already guard per node.

Five plugins demanded a selection they never used. Only HTML Renderer genuinely needs a parent: its factory seeds the new node's content from document.nodes[parent_node_id].content. The other five used parent_node_id as a place_child anchor and an edge, nothing more - so requiring a selection made the picker a dead end on an empty canvas.

Change

Failures on the node - artifact_error joins artifactContent on the wire, rendered in the card with the same banner treatment the sandbox uses. Live-wire only: research_error appears in neither session_save.py nor session_load.py, so a reloaded session does not resurrect a stale failure. Cleared when a new instruction is sent and when a turn lands.

Per-node runs - both kinds move onto the node.pending_request_id guard Gitlink and Code Sandbox established, and name the busy node. Two details that guard shape demands:

• The stamp moves out of _run() to immediately after the claim, and the WS wrapper claims a placeholder synchronously before its first await. The field is the gate now, not UI bookkeeping, and a value written after an await leaves a window where a second click on one node sees an idle node. Same mechanism as _GITLINK_RUN_CLAIM_PLACEHOLDER.
• Busy means "the registry still holds this run", not "the node has a request id". Cancelling releases the slot at once while the worker unwinds in its own time, and during that window the node still carries the dead id - reading the field alone refused the very next Run clicked after a cancel. AgentDispatcher.is_node_run_live asks the registry.

test_dispatch_claim_ordering.py's AST guard now covers both guard shapes rather than only the is_busy() one, so the no-await-between-guard-and-claim property stays enforced for the two converted kinds instead of silently going unchecked.

Parentless creation - make_simple_child_node_handler gains an opt-in standalone path that spawns at the picker's reported viewport centre and leaves the node unconnected. The five add_*_node methods already tolerated parent_id=None internally, so only their annotations widened. HTML Renderer is unchanged.

Also removes the Validation & Delivery picker category, which carried a full name and description, zero plugins, and was skipped at render.

Test plan

• Full backend suite on a clean checkout of this branch: 3092 passed, 19 skipped. ruff clean. python contracts/codegen.py --check: 13 artifact sets up to date after regeneration.
• npm run check (schema drift, typecheck, lint, vitest, build, bundle size): green.
• npx playwright test: 5/5 - the specs bootstrap through the Plugins picker, which the parentless change touches.
• New backend coverage: fail_artifact_generation records the message, no-ops on a deleted node, and is cleared by both a new instruction and a landed turn; two web-research nodes hold in-flight runs simultaneously; a second run on the same node is bounced without overwriting its query, with the node-named message; cancelling frees the node for an immediate re-run (the regression the liveness check exists for); each of the five plugins creates an unconnected node at the reported spawn point and treats a stale selection as no selection, while HTML Renderer still refuses.
• Per-builtin parent behavior is now pinned once, exhaustively, in test_plugin_builtin_migration.py's parametrized suite; ten near-identical per-plugin duplicates in test_plugins.py were replaced by a single test asserting the split itself.
• Frontend: the artifact card renders the failure banner with role="alert" and renders none when there is no error.