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/core/commands/variableCommands.ts

@@ -17,7 +17,7 @@ import {
   variableDef,
 } from '../properties/PropertySchema';
 import { type Removed, removeFirst, restoreAt } from '../util/list';
-import { type Command, CommandError } from './Command';
+import { type Command, CommandError, RenameCommand } from './Command';
 import { renameVariablePorts } from './portCommands';
 import { requireVariableOwner } from './require';
 
@@ -117,26 +117,16 @@ export class RemoveVariableCommand implements Command {
 }
 
 /** Renames everywhere: definition, default, instance overrides and the graph nodes referring to it. */
-export class RenameVariableCommand implements Command {
-  readonly label: string;
-
+export class RenameVariableCommand extends RenameCommand {
   constructor(
     private readonly assetId: string,
-    private readonly from: string,
-    private readonly to: string,
+    from: string,
+    to: string,
   ) {
-    this.label = `Rename variable ${from}`;
+    super(`Rename variable ${from}`, from, to);
   }
 
-  execute(project: Project): void {
-    this.rename(project, this.from, this.to);
-  }
-
-  undo(project: Project): void {
-    this.rename(project, this.to, this.from);
-  }
-
-  private rename(project: Project, from: string, to: string): void {
+  protected rename(project: Project, from: string, to: string): void {
     const asset = requireVariableOwner(project, this.assetId);
     if (from === to) return;
     if (!isValidVariableName(to)) throw new CommandError(INVALID_NAME);
@@ -316,27 +306,17 @@ export class RemoveStructFieldCommand implements Command {
   }
 }
 
-export class RenameStructFieldCommand implements Command {
-  readonly label: string;
-
+export class RenameStructFieldCommand extends RenameCommand {
   constructor(
     private readonly assetId: string,
     private readonly key: string,
-    private readonly from: string,
-    private readonly to: string,
+    from: string,
+    to: string,
   ) {
-    this.label = `Rename field ${from}`;
+    super(`Rename field ${from}`, from, to);
   }
 
-  execute(project: Project): void {
-    this.rename(project, this.from, this.to);
-  }
-
-  undo(project: Project): void {
-    this.rename(project, this.to, this.from);
-  }
-
-  private rename(project: Project, from: string, to: string): void {
+  protected rename(project: Project, from: string, to: string): void {
     const owner = requireFieldOwner(project, this.assetId, this.key);
     if (from === to) return;
     if (!isValidFieldName(to)) throw new CommandError(INVALID_FIELD);