Let variables, fields, ports and list items be dragged into order (#310)

Variables of a behavior, the fields of a struct, the ports of a start
or end node and the items of a list could only be added and removed.
Changing their order meant removing and adding them again. Now each of
them is dragged by the mark in front of its row.

- sortBlocks (ui/blockDrag.ts) makes the rows of one column sortable.
  A block is every row it covers, so a struct moves with its fields and
  a list item with its nested list. The block is dimmed while it is
  carried, an accent line shows where it lands, and a scrolled panel
  follows the pointer at its edge. Less than 4 px stays a click.
- The arithmetic is pure and lives in ui/sortable.ts: blockGap,
  gapLineY, dropSlot and edgeScroll. dropIndex of the tile strip moved
  there too, so the reorder maths sits in one file.
- MoveVariableCommand, MoveStructFieldCommand and MovePortCommand
  build on the existing MoveCommand. Undo moves back.
- Lists need no command. InlineList replaces the list as a whole, as
  it does for every other edit of its items.
- The grip is the kind mark, for a list item its number. Moving only
  happens among siblings: a field stays in its struct.
- On a Struct asset the move runs through WithStructTypes. Typed
  variables, Pack/Expand ports and split outputs follow the new order.
- Values and edges hang on names, so only the order changes. Nodes
  running a behavior read their ports from the start node, the new
  order reaches them without a copy.
- Not sortable by design: the variables of a placed object, the fields
  of a typed struct (moved on the Struct asset) and the picked ports
  of Variables/Properties nodes.
- Test hook __gm.inspectorMark(label) finds the mark of a row.

Tests:
- Gate on melektaus-atx: 235 files, 3572 tests, CHECK EXIT 0 (+9).
- tests/core/reorder.test.ts: a variable, a field two levels deep, a
  Struct asset reaching a typed variable and Pack, and start ports
  reaching the calling node with edges and result kept.
- e2e 22: ALL PASSED, 39 checks (+4, sum 1739). 22f-iii drags a
  variable past a struct of several rows, then a field inside it.
- Counter-probes: dropSlot by top edge instead of middle and a field
  move that ignores its path fail 3 unit tests. With sortBlocks
  switched off, the three order checks of 22f-iii fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/commands/variableCommands.ts

@@ -27,8 +27,8 @@ import {
   variableDef,
 } from '../properties/PropertySchema';
 import { valueAt, withValueAt } from '../properties/valuePath';
-import { type Removed, removeFirst, restoreAt } from '../util/list';
-import { type Command, CommandError, RenameCommand } from './Command';
+import { moveItem, type Removed, removeFirst, restoreAt } from '../util/list';
+import { type Command, CommandError, MoveCommand, RenameCommand } from './Command';
 import { renameVariablePorts } from './portCommands';
 import { requireBehavior, requireVariableOwner } from './require';
 
@@ -183,6 +183,22 @@ export class RenameVariableCommand extends RenameCommand {
   }
 }
 
+/** Only the order changes: values and the ports naming a variable go by its name (#310). */
+export class MoveVariableCommand extends MoveCommand {
+  constructor(
+    private readonly assetId: string,
+    from: number,
+    to: number,
+  ) {
+    super('Reorder variables', from, to);
+  }
+
+  protected move(project: Project, from: number, to: number): void {
+    if (!moveItem(requireVariableOwner(project, this.assetId).variables, from, to))
+      throw new CommandError(`There is no variable ${from + 1} to move to place ${to + 1}`);
+  }
+}
+
 /** The behavior, once it is known to declare `key`. */
 function requireBehaviorVariable(project: Project, assetId: string, key: string): BehaviorAsset {
   const asset = requireBehavior(project, assetId);
@@ -467,3 +483,25 @@ export class RenameStructFieldCommand extends RenameCommand {
       site.write(withFieldRenamed(site.read(), from, to));
   }
 }
+
+/** A struct value is read by field name, so only the declaration changes order (#310). */
+export class MoveStructFieldCommand extends MoveCommand {
+  /** `path` names the struct whose fields are reordered: none for the variable itself. */
+  constructor(
+    private readonly assetId: string,
+    private readonly key: string,
+    from: number,
+    to: number,
+    private readonly path: readonly string[] = [],
+  ) {
+    super('Reorder fields', from, to);
+  }
+
+  protected move(project: Project, from: number, to: number): void {
+    const owner = requireFieldOwner(project, this.assetId, this.key, this.path);
+    const fields = [...owner.fields()];
+    if (!moveItem(fields, from, to))
+      throw new CommandError(`There is no field ${from + 1} to move to place ${to + 1}`);
+    owner.setFields(fields);
+  }
+}