Mark what is in focus, so a frame can be judged without zooming to 100%

FR-CULL-3's focus peaking. One compute dispatch measures local contrast
in WGSL and writes an overlay texture; on desktop it reaches Slint
through the same zero-copy wgpu import the canvas uses, so nothing
per-pixel touches the CPU on the frame path.

With peaking off the cost is zero and structurally so: focus_overlay
opens with `let settings = self.peaking?;` before the frame is touched,
and clearing drops both overlay textures, so no VRAM is held either.

NFR-P14 is met by construction rather than by measurement -- one
dispatch, no second render, no pipeline compile after session open, and
a test asserting allocations stay at 2 over eight frames. The budget
test asserts 50ms at 4K rather than a tight bound, deliberately: a tight
bound fails on a loaded machine and gets deleted, which is worse than a
loose one that still catches the regression that matters.

TD-1 is amended rather than joined by a TD-6: on Android the overlay
rides the readback that already exists there, roughly doubling that
transfer while peaking is on, and TD-1's own "Done when" removes both
because both are the same missing capability.

Verified: cargo fmt clean; clippy --workspace --all-targets -D warnings
green, which also compiles peaking.slint through dr-ui's build.rs; 11
focus GPU tests and 79 baseline dr-gpu tests pass; 511 dr-ui tests pass.
Not verified: the cfg(target_os = "android") arm, which the host-target
clippy never compiled.

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

core/dr-gpu/src/lib.rs

@@ -21,6 +21,7 @@ mod adjust;
 mod demosaic;
 mod detail;
 mod error;
+mod focus;
 mod histogram;
 mod mask;
 mod readback;
@@ -33,6 +34,7 @@ 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};