Snap to the grid from the start in every graph tab (#391)

Laying out a graph by hand mostly means lining nodes up, so the
magnet was switched on in nearly every session. Every graph tab now
starts with it lit: Behavior, State machine, Shader, Particles,
Sound FX, Instrument, the song's Routing and the Routing tab.

- SnapToggle takes the state it starts in and lights its button to
  match. GRAPH_SNAPS_AT_FIRST sets it for the toggles and for
  GraphCanvas alike, so button and canvas cannot disagree.
- Curve and Vector Art are no graphs and still start without it.
- New hook __gm.graphSnapping(on?) reads every graph tab's magnet,
  or "mixed" where button and canvas differ, and switches them all.
- The e2e harness reads the magnets after each navigate() and then
  switches them off, as it already sets the zoom transition to 0.
  Most checks drag a node to an exact spot. e2e 45 does the same
  itself, because the editor comes up there through the sign-in.
- e2e 06 checks that all eight graph tabs started with the magnet on.
  The checks in 06, 26 and 65 that read it off now say they read
  what the harness left.

Tests:
- Unit snap-toggle: 4 tests, 1 new (lit at first, nothing asked of
  the target before a click).
- e2e 06 (melektaus-atx): 150 checks, 1 new. e2e 34: 134 checks.
- Gate (melektaus-atx, together with #390): 287 files, 4397 tests.
- Pictures looked at: none, the state comes from the hook.

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

tools/lib/harness.mjs

@@ -491,7 +491,14 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
     check('debug hook present', Boolean(await evaluate('Boolean(window.__gm?.state)')));
     // The zoom is shown as a transition (#309); a check reads the step it lands on, not the way.
     await evaluate('__gm.zoomTransition?.(0)');
+    // Every graph snaps from the start (#391); the checks drag nodes to exact spots, so the magnets
+    // go off here. What they started at stays readable for the check that wants it.
+    snapAtFirst = (await json('__gm.graphSnapping?.() ?? null')) ?? null;
+    await evaluate('__gm.graphSnapping?.(false)');
   };
+  /** Each graph tab's magnet as the last load left it, before the harness switched it off (#391). */
+  let snapAtFirst = null;
+  const graphSnapAtFirst = () => snapAtFirst;
 
   return {
     cdp,
@@ -545,6 +552,7 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
     signIn,
     sceneOf,
     navigate,
+    graphSnapAtFirst,
   };
 }