Merge: focus peaking, so a frame can be judged without zooming to 100%

FR-CULL-3's peaking half. The raw histogram and raw clipping indicators
remain unbuilt -- what exists is a display histogram tagged FR-DSP-7,
counting AdjustPass's 8-bit output, which reports a highlight as gone
precisely where FR-CULL-3 needs it to report the highlight recoverable.

Verified before merge: fmt clean, clippy --workspace --all-targets
-D warnings green, 11 focus GPU tests, 79 baseline dr-gpu tests, 511
dr-ui tests. The cfg(target_os = "android") arm is unverified -- the
host-target clippy never compiled it.

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

# Conflicts:
#	ui/dr-ui/src/lib.rs
#	ui/dr-ui/ui/app.slint

docs/technical-debt.md

@@ -55,6 +55,27 @@ readback is at viewport resolution, not sensor resolution. The 7.43 ms at 4K in
 not the bill. **It has not been measured on the device**, which is the first thing to do if the
 develop view feels heavy on the tablet; do not assume this is the cause without a number.
 
+### And a second transfer, while focus peaking is on
+
+Added 2026-08-29 with FR-CULL-3. The focus-peaking overlay is a compute pass writing its own
+`Rgba8Unorm` texture, which on desktop reaches the compositor with no copy — but on Android there is
+no more a path for *that* texture than for the frame it belongs to, and an overlay that stayed on
+the device while the picture underneath it did not would simply never be seen. So
+`FocusPeakPass::read_overlay` follows the frame back through memory, and the Android frame path
+carries **two** full-resolution `copy_texture_to_buffer` transfers instead of one.
+
+This is recorded under TD-1 rather than as its own entry because it is not an independent choice.
+It exists only because TD-1 exists, it is bounded by the same thing — `render` fits the pass to the
+canvas, so both transfers are at viewport resolution — and TD-1's "Done when" already covers it:
+whichever of the three fixes above lands removes the readback for the frame and the overlay
+together, because both are the same missing capability.
+
+Two things worth saying plainly. The doubling is **reasoned, not measured on the device** — the same
+gap TD-1 admits about its own cost, and the reason neither number should be quoted as a measurement.
+And it is paid only while the photographer has the overlay switched on: `DevelopSession::focus_overlay`
+returns on its first line when peaking is off, so with it off there is no dispatch and no transfer,
+and the Android frame path is exactly what it was before this feature existed.
+
 ### Paying it off
 
 Any one of these removes it: