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/tile/PictureStrip.ts

@@ -8,6 +8,7 @@ import type { TileAsset } from '../../core/model/Asset';
 import { pictureChoices, textureRefLabel } from '../../core/properties/textureRef';
 import {
   Button,
+  dropIndex,
   type DropSpot,
   HStack,
   Label,
@@ -22,24 +23,6 @@ import type { EditorState } from '../state';
 
 const WIDTH = 210;
 
-/**
- * Where a dragged row lands, counted in the list it has *left* – which is what the move command
- * takes. A drop on the middle of a row takes that row's place: the list is flat, so there is no
- * "inside" for it to mean, and without this a drag downwards onto the next row would cancel itself
- * out. The gaps above and below a row are insert points, and one below the dragged row shifts up
- * by one once the row is out. `-1` is "nothing to do".
- */
-export function dropIndex(from: number, spot: DropSpot, length: number): number {
-  const last = length - 1;
-  if (from < 0 || from > last) return -1;
-  if (spot.at === 'end') return last;
-  const target = Number(spot.targetId);
-  if (!Number.isInteger(target) || target < 0 || target > last) return -1;
-  if (spot.at === 'onto') return target;
-  const insert = target + (spot.at === 'after' ? 1 : 0);
-  return Math.min(last, insert > from ? insert - 1 : insert);
-}
-
 /**
  * The picture list beside the board (#118): one row per animation step, in the order they run.
  * Rows are addressed by their place rather than by the picture they name – the same block may be