Two compilers that walk a graph the same way walk it through one emitter (#85)

Refactoring round two: the particle and shader compilers share emitGraph.ts (cycle detection,
duplicate names, emit order) and keep only their rules. FontLibrary and TextureLibrary sit on
FileLibrary (base URL, loading/failed bookkeeping, listeners); SoundLibrary keeps its own
contract. The three Create*Command share ImportFileCommand, the three renames share
RenameCommand - class names stay. bandBounds() in framing.ts and portNameField() in
portSection.ts replace in-file clones, and three comments that told history now state the rule.

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

src/ui/widgets/Popup.ts

@@ -28,8 +28,8 @@ export function screenMargin(screenHeight: number): number {
 /**
  * Where a popup of `height` ends up when it was asked to open at `wanted`. The margin has to be
  * enforced here, not only in `roomFor`: a popup anchored low is *shifted* up to fit, and shifting
- * it against the raw canvas height parks it on the bottom edge – which is why the gap used to be
- * visible above and missing below.
+ * it against the raw canvas height would park it on the bottom edge, with the gap above it and none
+ * below.
  *
  * Downwards only, though. Above the popup the margin is what the height cap spends, not a distance
  * to keep: a menu hanging off a button near the top of the screen belongs under its button, and