Find hot photosites without repairing them, and measure a sensor's aging

The hot-pixel pass could only repair: it returned how many photosites it
changed and threw away which. find_hot_pixels runs the same pass and
returns them as sensor coordinates, leaving the frame alone, so a sensor's
defects can be tracked across frames.

sensor_scan prints each frame's candidates, and with --probe reads a list
of coordinates back out of every frame. Run over 53 6D raws from 2015 to
2026, it found 32 persistent defects, 2 in 2015 and 32 by 2026, and showed
what a defect map has to account for: a frame that does not flag a
photosite proves nothing unless its neighbourhood is dark, and the 6D
hides some of its defects itself above ISO 5000. docs/dev/sensor-health.md
records the findings and the design they argue for.

core/dr-gpu/src/lib.rs

@@ -38,7 +38,7 @@ pub use adjust::AdjustPass;
 // rather than an implementation detail: a detail pass is guaranteed linear,
 // unclipped, full internal precision (FR-DEV-2), and anyone reasoning about
 // VRAM at 24 MP needs to know what an intermediate costs.
-pub use demosaic::{DemosaicedImage, Demosaicer};
+pub use demosaic::{DemosaicedImage, Demosaicer, Photosite};
 pub use detail::INTERMEDIATE_FORMAT as DETAIL_INTERMEDIATE_FORMAT;
 pub use error::GpuError;
 pub use focus::{FocusPeakPass, FocusPeaking, PeakColour, PeakSensitivity};