docs(mapgen-studio): add pipeline DAG stage-view workstream record, proposal, design, spec deltas, and tasks

[mateicanavra]

This workstream establishes the planning and specification foundation for integrating the recipe DAG visualization as a first-class Pipeline stage view within the redesigned Studio shell.

The merged DAG feature (PRs #1587–#1591) was built against the pre-redesign monolith — hard-coded palette forks, monolith-owned view state, and chrome mounted through the deleted App.tsx. This change lays out exactly how that surface gets re-expressed natively in the redesign without touching any of its semantic contracts.

What the implementation slice will deliver:

• A floating Map | Pipeline segmented switcher at the top edge of the stage area — stage furniture, not Game bar or World console content — using the established popover-tier pill idiom.
• PipelineStage, a token-driven replacement for RecipeDagView that preserves every interaction semantic from the handoff verbatim: stage selection vs. step expansion, click-again unselect, per-artifact connector label selection with trunk-before-fan-out projection, focus dimming with active elements rising, domain-driven accents, and the diagnostics panel.
• The map canvas stays mounted (hidden) when the Pipeline view is active so camera state and in-flight generation runs are unaffected. The Explore dock hides in Pipeline view; the Recipe dock, Game bar, and World console persist.
• useRecipeDagQuery on TanStack Query replaces hand-rolled fetch/cancellation state — lazy fetch on first activation, cached per recipe ID, typed error surface.
• stageView, pipelineSelectedStageId, and pipelineExpandedStageIds land in viewStore (Zustand, browser-only). Connector-label selection stays component-local as in the merged code.
• RecipeDagView.tsx is deleted; its semantic test assertions are ported onto PipelineStage.test.tsx in the same change.

Preserved untouched: layout.ts, domainPresentation.ts, artifactPresentation.ts, client.ts, all server-side recipeDag modules, the oRPC schema, the vite middleware, and all mapgen-core recipe-dag projection logic and tests.

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[mateicanavra]

Merge activity

• Jun 12, 8:50 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jun 12, 10:14 PM UTC: Graphite rebased this pull request as part of a merge.
• Jun 12, 10:14 PM UTC: @mateicanavra merged this pull request with Graphite.