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]>

docs/CURRENT-STATE.md

@@ -2792,7 +2792,11 @@ one provider it is the layer switch too, and with several, darkening the last
 lit chip turns the layer off while leaving that provider switched on. The
 viewer sits first, under the header, with its tools (EXPAND, FIT/FILL, FOLLOW,
 close) in a bar above the image so the provider's own overlays stay clear;
-the settings follow. SINCE is a stepped range input (any date, 10, 5, 3, 2 and
+the settings follow. FOLLOW is offered only while the map stack is Google 3D
+(`photoreal`): the layer takes the application map stack controller through
+`attachMapStackController`, subscribes to stack switches, disables the button
+with a MAP SOURCE hint on other stacks, and stops following when the stack
+leaves Google 3D. SINCE is a stepped range input (any date, 10, 5, 3, 2 and
 1 years, 6, 3 and 1 months) whose readout names the cut-off date. The panel is
 portable (`street-level-panel` spec in `PanelPositionControls`, minimum
 320 × 280, `dockOnCollapse`): in a resized window the viewer takes the spare