Split the object inspector, and sweep out the section banners

objectSection.ts had grown to 559 lines around three separable parts: the
queries, the vector rows and the parent row. Four object commands repeated the
same capture/restore preamble – applyCommands stays out, its five carry extra
undo state whose restore order matters.

58 dashed banner comments are gone. The nine labels in nodes/index.ts stay:
they are the only structure a flat 90-entry array has.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/runtime/systems/Input.ts

@@ -76,7 +76,6 @@ export class Input {
     return this.released;
   }
 
-  /** Keys currently held. */
   get heldKeys(): ReadonlySet<string> {
     return this.down;
   }