Replace the five variable nodes with one Variables node

A single node now lists any number of variables. Every entry is an output
that reads the current value whenever it is pulled; an entry switched to
Set is also an input and is written when the flow arrives. Exec pins only
exist while at least one entry writes, and all inputs are read before any
write, so one node can swap two variables.

Get/Set variable, Get/Set variables and Variable are gone. Old project
files are converted on load: each old node becomes a Variables node with
the same variables, edges and typed values.

A written port without a typed value now shows the zero of its kind on
the node body, matching what the inspector shows and what the run writes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/lib/builders.mjs

@@ -42,9 +42,12 @@ export const node = (id, type, x, y, params = {}, ports) => ({
   ...(ports ? { ports } : {}),
 });
 
-/** A port a behavior's start or end node declares. */
+/** A port a node declares: an input of a start node, a field of a struct, a variable read by `Variables`. */
 export const port = (key, kind) => ({ key, kind });
 
+/** A variable a `Variables` node writes as well as reads. */
+export const written = (key, kind) => ({ key, kind, write: true });
+
 /**
  * A behavior variable declaration. `items` is the item kind of a `list`, `fields` the shape of a
  * `struct` – or of the items of a `list` whose items are structs.