feat(street-level): camera follow only on the Google 3D map

FOLLOW puts the globe camera at eye height inside the street-level view; on
flat imagery or terrain there is nothing to stand among. The layer now takes
the application map stack controller (attachMapStackController), subscribes
to stack switches, offers FOLLOW only while the stack is 'photoreal', disables
the button with a MAP SOURCE hint elsewhere, and stops following when the
stack leaves Google 3D. Selection uninstall is a no-op when never installed.

The QA gate checks the disabled path and, when Google 3D is available, that
leaving it stops following; its caption-timeout dump now forces one render to
tell a missed update from a render fault.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/layers/streetLevel/policy.js

@@ -22,5 +22,12 @@ export const FILTER_DEFAULT = Object.freeze({ pano: 'all', sinceDays: 0 });
 /** Nearest-image search when the user asks to look at a place. */
 export const NEAREST_RADIUS_M = 50;
 
+/**
+ * Map stack the globe camera may follow the street-level view on. Only Google
+ * Photorealistic 3D has buildings to stand among; on flat imagery or terrain
+ * a camera at eye height looks at a smeared texture.
+ */
+export const FOLLOW_MAP_STACK_ID = 'photoreal';
+
 /** Eye height above the sampled ground when the globe camera follows the viewer. */
 export const FOLLOW_EYE_HEIGHT_M = 2.4;