feat: Scene | Runtime tabs in the graph during play mode (closes #898)

[marquezmiguel]

Summary

Implements #898: while playing, the graph now shows two tabs following the inspector's tabs pattern — Scene keeps the edited scene fully editable exactly as before (undo/redo, context menu, drag and drop, save), and Runtime shows the live hierarchy of the play scene: objects created by scripts appear as they spawn, can be selected and inspected, and tweaked through the inspector (volatile changes, lost on stop). This mirrors the Local/Remote workflow of other editors for data-driven games where most content is spawned at runtime.

When the game is not playing, behavior is byte-equivalent to the current editor: no tabs, no guards, nothing changes.

Changes made

• Graph: dual tree state (edited scene / play scene) with tabs rendered only while playing. Separate trees keep id collisions between the exported scene and the play scene from leaking selection or expansion states across tabs.
• Selection/expansion preservation across refreshes is now indexed by id (Map) instead of scanning the previous tree per node: runtime scenes can hold tens of thousands of nodes and the previous O(n²) matching froze the UI (measured 46s → ~0.2s per refresh on a 22k-node scene).
• Structural operations (rename, reparent, remove, clone, paste, drops, enabled toggles) are guarded per object: play scene objects are read-only, edited scene objects keep the exact current behavior even while playing.
• Undo/redo: registerUndoRedo items now carry an optional object used by a volatile predicate installed during play — edits on edited scene objects keep recording history, edits on play scene objects execute without polluting the stack (they would reference disposed objects after stop).
• Play lifecycle hardening: re-entrance guards on restart (double restart / src watcher / ipc triggers), stop-during-loading and stop-during-restart are now safe, and forceCompileAllSceneMaterials resolves when its scene is disposed mid-compilation instead of hanging forever.
• Scene inspector: global sections (physics gravity, rendering pipelines) are hidden when inspecting the play scene — they edit the edited scene and would silently persist without undo.
• New tools/scene/play/runtime.ts helpers (isScenePlaying, getObjectScene, isPlaySceneObject) shared by all the guards, with unit tests. Existing behavior is preserved by 72 passing tests.

Benefits

• The editor becomes usable for data-driven / procedurally-loaded games (worlds loaded from data at runtime), where today the graph shows nothing of what actually runs.
• Runtime objects can be selected and inspected live (materials, transforms) like in other engines' play modes.
• No behavior change at all outside play mode, and the edited scene can still be worked on while the game runs.

Follow-ups (out of scope, happy to discuss)

• Viewport picking of runtime objects while playing.
• Gizmos attached to runtime objects (requires a utility layer on the play scene).
• Structural editing of the play scene (Unity-like volatile hierarchy edits) and an "apply play changes to scene" action.

Note for reviewers: future registerUndoRedo call sites should pass object when reachable during play; items without it fall back to the inspector's edited object to classify volatility.

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