Give the scene tab a header with Fit and the eye (#245)

The scene view had no header bar of its own, and its eye sat in the
top bar. Switching between Scene and Game also showed two different
pictures, since nothing tied the free view to the camera.

- Theme.headerPadding (4) replaces Theme.padding in the top bar and
  in TabHeader, for the padding and for minHeight. Both bars shrink
  from 40 to 32 px. Nothing else uses the new value.
- The new ScenePane wraps the scene view in a header bar with Fit
  and the visibility eye. The eye is gone from the top bar, which
  keeps the menu, undo and redo.
- Fit with an active camera shows the world exactly as the Game tab
  does at its chosen step: the same zoom, and the camera centre on
  the same screen pixel. The rule is the pure freeViewOfCamera in
  viewport/view.ts, which also mirrors the letterbox snapping.
- Fit without a camera frames all content on a whole step, as
  before.
- New hook __gm.sceneHeaderAt(name). e2e 46g opens the eye there.
- The doc comment above Theme.treeIndent went away in the working
  tree along with this. It is committed as found.

Not checked: the two views only match while they have the same size.
With the tilemap panel open the scene view is narrower, and Auto may
pick a smaller step there.

Tests:
- ./check.sh: 178 files, 2575 tests, CHECK EXIT 0
- 2 new unit tests in view.test.ts: three cameras times four steps
  land on the same screen point as in the Game tab, Auto stays whole
- e2e 3h (2 checks): a real click on Fit at Auto and at Fit, crate,
  camera frame and view box match in both tabs
- GM_STEPS=1-3,46: ALL PASSED, 94 checks
- Counter-probes: without the centre offset the unit test fails,
  with Fit ignoring the camera both 3h checks fail

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

src/editor/debug/sceneHooks.ts

@@ -9,7 +9,7 @@ import { centreOf, type HookContext } from './hookHelpers';
 
 /** The Scene and Game tabs, the tilemap painted in the scene view, and the Container tab. */
 export function sceneHooks({ state, editor }: HookContext) {
-  const { viewport, gameView, gamePane, containerEditor } = editor;
+  const { viewport, gameView, gamePane, scenePane, containerEditor } = editor;
   /**
    * A holder's objects the way the editor sees them: the content of placed containers is resolved
    * from the template, so it is nowhere in `holder.objects` (see `model/instance.ts`).
@@ -47,6 +47,9 @@ export function sceneHooks({ state, editor }: HookContext) {
       }
       return null;
     },
+    /** Where to click Fit or the eye in the scene tab's header. */
+    sceneHeaderAt: (name: 'fit' | 'visibility') =>
+      centreOf(name === 'fit' ? scenePane.fitButton : scenePane.visibilityButton),
     viewportInfo: () => viewport.debugInfo(),
     /** A full redraw of the Scene tab – what a pan, which only moves the camera, is compared with. */
     viewportRedraw: () => viewport.redraw(),
@@ -66,7 +69,7 @@ export function sceneHooks({ state, editor }: HookContext) {
     sceneIcon: (id: string) => viewport.iconInfo(id),
     /** The names shown beside the symbols, each with the plate under it, in world units and on screen. */
     sceneLabels: () => viewport.labelInfo(),
-    /** A collider's outline in the Scene tab: hidden by the eye in the top bar, it is neither seen nor hit. */
+    /** A collider's outline in the Scene tab: hidden by the eye in its header, it is neither seen nor hit. */
     sceneShape: (id: string) => viewport.shapeInfo(id),
     /** The switches behind the eye while it stands open: per type and part, on, enabled, where to click. */
     visibilityPopup: () =>