feat(steps): the rail — a handler read top to bottom, forks and all

The reading the walk records now has a picture. `#/steps?…&view=order` draws
the anchor, then its body: a box per step in the order the code writes them, a
fork where the code forks with its arms side by side under the condition, a
helper drawn where it is called, and an arm that answers, returns or throws
ending there — so proshop's login reads *look the user up · if the password
matches, sign a token inside the reply and answer 200 · otherwise 401*, which
is what the code says and what a row of four boxes could not.

- `program-model.ts` decides the words: the fork carries the decision once and
  its arms say only which side they are (WHEN / WHEN NOT), except a `switch`,
  whose arms each have a case to say, and a `try`, which says `on error` once.
- `StepBox.svelte` is the box both readings draw — the canvas wraps it in
  handles, the rail lets it size to its words. Same look, same click, same
  double-click-to-start-here.
- `StepsKey.svelte` is the key, floating over the canvas as before and last in
  the document on the rail, which scrolls and cannot have things sitting on it.
- The reading travels in the URL (`view=order` / `view=tree`) and the summary
  offers both; without one, the answer's own default decides — the code's order
  for a handler or an endpoint, the tree for a screen.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01REFyW9hmNrxhwN5wxRoAkC

ui/src/lib/wire.ts

@@ -814,6 +814,8 @@ export type WireArmEnd = 'reply' | 'return' | 'throw' | 'exit';
 export interface WireArm {
   /** This arm's own condition, in the words the rest of the view uses. */
   when: string;
+  /** The arm taken when the fork's condition does NOT hold — the `else` side. */
+  not?: true;
   /** How it leaves: it answers the request, returns, or throws. Null = it runs on. */
   ends: WireArmEnd | null;
   body: WireBlock;
@@ -836,7 +838,7 @@ export type WireItem =
    * after this function returns (`later`), or calls started together
    * (`together`).
    */
-  | { kind: 'block'; block: 'inline' | 'loop' | 'later' | 'together'; label: string; via?: WireNodeRef; within?: string; body: WireBlock; again?: true }
+  | { kind: 'block'; block: 'inline' | 'loop' | 'later' | 'together'; by?: string; via?: WireNodeRef; within?: string; body: WireBlock; again?: true }
   /** Where the reading stopped: a helper that calls itself, or a cap the walk hit. */
   | { kind: 'cut'; why: 'folded' | 'depth' };