Give memory back in the order the user will miss it least

FR-PLAT-AND-5. Android asks for memory back through onTrimMemory and
kills the process if it is not given; until now nothing listened, so the
answer was always "no".

A tiered registry answers instead: GPU caches first, then proxies, then
thumbnails, driven from android_main on MainEvent::LowMemory and
MainEvent::Stop. The order is the argument. A backgrounded app has no
window to draw and therefore no use for a render pipeline, while its
thumbnails are exactly what the user will be looking at half a second
after they come back -- so going into the background frees only the GPU
tier, and only being measured against death frees everything.

Sinks register beside the cache they free and hold weak handles, so the
registry cannot keep a controller -- and every decoded portrait in it --
alive past the interface it belonged to. `try_borrow_mut` and skip: a
warning can land mid-render, freeing textures under the code drawing
with them is worse than missing one, and a warning not acted on is
always followed by another.

The GPU test is the one that matters: an eviction must change no pixel.
A freed intermediate pool whose `colour_key` promise still stands
renders an empty texture, and nothing else would have caught it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/dr-ui/src/identity_ui.rs

@@ -105,6 +105,21 @@ impl IdentityController {
     fn clear_picks(&self) {
         self.picked.borrow_mut().clear();
     }
+
+    /// TRACES: FR-PLAT-AND-5 | NFR-RES-1
+    /// Drop the decoded rail portraits.
+    ///
+    /// The one in-memory image cache in this crate that is unbounded by
+    /// anything but the library: one decoded portrait per person, kept for as
+    /// long as the person exists. On a library with a few hundred named people
+    /// that is worth tens of megabytes of nothing but a saved decode.
+    ///
+    /// Costless to lose. `refresh` rebuilds any portrait it does not find, so
+    /// the only consequence is the JPEG decode this cache exists to skip, and
+    /// only for the people the rail is actually showing at the time.
+    pub fn clear_covers(&self) {
+        self.covers.borrow_mut().clear();
+    }
 }
 
 /// Push the people rail and the face grid into the window.