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/editor/debug/uiHooks.ts

@@ -54,7 +54,9 @@ export function uiHooks({ app, state, editor }: HookContext) {
     return control && { ...centreOf(control), left: control.getGlobalPosition().x };
   };
   /** The control right of the row label `label`, as `fieldOf` finds it. */
-  const controlOf = (label: string, last: boolean): Widget | null => {
+  const controlOf = (label: string, last: boolean): Widget | null => besideOf(label, last, 1);
+  /** The widget `step` places along from the row label `label`: +1 its control, -1 its mark. */
+  const besideOf = (label: string, last: boolean, step: number): Widget | null => {
     let found: Text | null = null;
     const walk = (node: Container): void => {
       if ((found !== null && !last) || !node.visible) return;
@@ -70,7 +72,7 @@ export function uiHooks({ app, state, editor }: HookContext) {
     while (owner && !(owner instanceof Widget)) owner = owner.parent;
     const row = owner?.parent;
     if (!row || !owner) return null;
-    const control = row.children[row.children.indexOf(owner) + 1];
+    const control = row.children[row.children.indexOf(owner) + step];
     if (!(control instanceof Widget)) return null;
     // Taller than the window since its groups have bands (#290): the row is scrolled into view.
     inspector.reveal(control);
@@ -164,6 +166,11 @@ export function uiHooks({ app, state, editor }: HookContext) {
       return (last ? hits.at(-1) : hits[0]) ?? null;
     },
     inspectorField: fieldOf,
+    /** The mark in front of the row named `label` – what a variable, a field or a port is dragged by (#310). */
+    inspectorMark: (label: string, last = false) => {
+      const mark = besideOf(label, last, -1);
+      return mark && centreOf(mark);
+    },
     /** The eyedropper beside the object field of a row (#276): where to press it, and whether it is lit. */
     inspectorEyedropper: (label: string, last = false) => {
       const button = controlOf(label, last)?.children.at(-1);