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

core/dr-gpu/src/lib.rs

@@ -22,6 +22,7 @@ mod adjust;
 mod demosaic;
 mod detail;
 mod error;
+mod focus;
 mod histogram;
 mod mask;
 mod readback;
@@ -34,6 +35,7 @@ pub use adjust::AdjustPass;
 pub use demosaic::{DemosaicedImage, Demosaicer};
 pub use detail::INTERMEDIATE_FORMAT as DETAIL_INTERMEDIATE_FORMAT;
 pub use error::GpuError;
+pub use focus::{FocusPeakPass, FocusPeaking, PeakColour, PeakSensitivity};
 // Renamed on the way out: `BINS` says enough inside `histogram`, and nothing
 // at all at a crate root shared with demosaic and segmentation.
 pub use histogram::{Histogram, HistogramPass, BINS as HISTOGRAM_BINS};