Let clarity's base be computed where it is still fully determined

Clarity's Gaussian sigma is 1.2% of the frame's shorter edge, so its radius
is a property of the viewport: 52 render pixels at 4K, two separable passes
of 105 taps each over 8.3 M pixels. That measured 33.9 ms — seven times the
entire fused point chain, for one slider — and is docs/technical-debt.md TD-4.

A detail pass may now declare `output_scale`, and clarity's base is computed
on a grid a quarter the size on each axis.

The pass that combines needs the blur *and* the full-resolution colour, and a
colour that has been through a quarter-scale target is no longer full
resolution. So a scaled pass cannot simply join the ping-pong: there are two
chains now. The full-resolution one carries the colour and no scaled pass
touches it; the reduced one carries the base and reaches the combining pass
through a second binding as `reduced_at()`.

The reduce is a dispatch of its own rather than something the first blur half
does on the way past, and that is the whole difference between this and the
strided kernel the module documentation rules out. A stride samples an image
that is not band-limited and aliases high-frequency content down into the
base, which is then subtracted, and arrives in the output as mottling across
smooth gradients. This band-limits first and samples after. What is discarded
is content the base could not represent at any resolution, because a Gaussian
at sigma = 26 px holds nothing above one cycle per 26 px and the quarter-scale
grid carries one per 8 — so the reduced base is not an approximation of the
full-resolution one, it is the same function sampled where it is still
determined.

Which is also why the scale belongs to the band rather than to the stage.
Texture's sigma is a decade finer, so the reduce pass's own box would be wider
than the Gaussian it was prefiltering; texture never reduces. And clarity
steps 4 -> 2 -> 1 as sigma falls, because a quarter of a small sigma is not a
Gaussian either — the case that gives up is the one that was already cheap.

`radius` stays in each pass's own pixels and `ComposedDetail::radius` multiplies
it back up, so 13 reduced pixels at scale 4 still report the 52 render pixels a
tile would have to be grown by. The halo a scheduler sees does not move.

The halo tests pass unchanged, which was TD-4's stated bar; they render at
1024 px and so exercise the reduced path rather than stepping around it. Added
`crossing_the_reduction_threshold_does_not_change_the_picture`, because
nothing yet compared the reduced form against a *less* reduced one — every
other test measures one form against itself. It renders the same edit either
side of the 4 -> 2 step-down and holds the peak excursion to 0.03 stops and
the reach to 2% of the frame.

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

core/dr-gpu/tests/local_contrast.rs

@@ -27,6 +27,7 @@
 
 use dr_gpu::{AdjustPass, DemosaicedImage, GpuContext};
 use dr_pipeline::descriptor::{OpId, ParamId};
+use dr_pipeline::detail::RenderScale;
 use dr_pipeline::ops::local_contrast::Clarity;
 use dr_pipeline::{Affects, EditGraph, OutputMode};
 use dr_types::ColourSpace;
@@ -323,6 +324,87 @@ fn a_proxy_and_an_export_agree_about_the_effect() {
     );
 }
 
+#[test]
+fn crossing_the_reduction_threshold_does_not_change_the_picture() {
+    // TRACES: FR-DSP-3 — `docs/technical-debt.md` TD-4, held in pixels.
+    //
+    // Clarity's base is computed on a reduced grid, and how reduced depends on
+    // the viewport: `LocalContrast::reduction` steps 4 -> 2 -> 1 as sigma
+    // falls, because a quarter of a small sigma is not a Gaussian any more.
+    // The whole claim of the optimisation is that this is invisible — that a
+    // base sampled at a quarter is not an approximation of the full-resolution
+    // one but the same band-limited function, sampled where it is still fully
+    // determined.
+    //
+    // Every other test in this file measures one form against itself. This is
+    // the only one that measures the forms against *each other*, and it is the
+    // one that would fail if the reduction were quietly softening the control,
+    // shifting it half a reduced pixel, or blocking the base into 4x4 squares.
+    //
+    // The two sizes are chosen to sit either side of a step-down: sigma is
+    // 1.2% of the shorter edge, so 512 gives 6.1 px and reduces by four, while
+    // 288 gives 3.5 px — under `MIN_REDUCED_SIGMA` once quartered — and
+    // reduces by two. Asserted rather than assumed, because the whole test is
+    // vacuous if both sides land on the same reduction.
+    let Some(ctx) = ctx() else { return };
+    assert_eq!(
+        Clarity::with_amount(100.0).reduction(RenderScale::full((512, 512))),
+        4
+    );
+    assert_eq!(
+        Clarity::with_amount(100.0).reduction(RenderScale::full((288, 288))),
+        2
+    );
+
+    let source = step_edge(&ctx, SIZE, [1.0, 1.0, 1.0]);
+
+    // Peak excursion in stops and the fraction of the frame it covers — the
+    // same two numbers `a_proxy_and_an_export_agree_about_the_effect` uses,
+    // and for the same reason: both are scale-free, so they are comparable
+    // between two renders of different sizes.
+    let measure = |out: u32| -> (f32, f32) {
+        let mut plain_pass = AdjustPass::new(&ctx);
+        let plain = row(
+            &render(&mut plain_pass, &EditGraph::default_chain(), &source, out),
+            out,
+            out / 2,
+        );
+        let mut pass = AdjustPass::new(&ctx);
+        let edited = row(
+            &render(&mut pass, &graph_with(CLARITY, 100.0), &source, out),
+            out,
+            out / 2,
+        );
+        let moved: Vec<f32> = (0..out as usize)
+            .map(|x| stops(edited[x], plain[x]).abs())
+            .collect();
+        let peak = moved.iter().cloned().fold(0.0f32, f32::max);
+        let touched = moved.iter().filter(|m| **m > peak * 0.1).count();
+        (peak, touched as f32 / out as f32)
+    };
+
+    let (quartered_peak, quartered_reach) = measure(512);
+    let (halved_peak, halved_reach) = measure(288);
+
+    // The same tolerances the proxy/export test uses. They are not loose: the
+    // bound this operation guarantees is 0.35 stops, so 0.03 is under a tenth
+    // of the full excursion.
+    assert!(
+        (quartered_peak - halved_peak).abs() < 0.03,
+        "a quarter-scale base gives {quartered_peak:.3} stops and a half-scale \
+         one {halved_peak:.3}; the reduction is supposed to be invisible"
+    );
+    assert!(
+        (quartered_reach - halved_reach).abs() < 0.02,
+        "the effect covers {quartered_reach:.3} of the frame reduced by four \
+         and {halved_reach:.3} reduced by two; the base has changed width"
+    );
+    assert!(
+        quartered_peak > 0.1,
+        "{quartered_peak:.3} stops — two flat images would also agree"
+    );
+}
+
 #[test]
 fn texture_acts_at_a_finer_scale_than_clarity() {
     // The whole reason there are two nodes. If the two controls ever reach the
@@ -485,10 +567,15 @@ fn dragging_the_slider_re_runs_the_detail_stage_and_nothing_else() {
     let mut graph = graph_with(CLARITY, 40.0);
     render(&mut pass, &graph, &source, 256);
     assert_eq!(pass.colour_dispatches(), 1);
+    // Four at this size: reduce, the two blur halves on the reduced grid, and
+    // the combine. σ is 1.2% of 256 px, so `LocalContrast::reduction` lands on
+    // a half here rather than the quarter a desktop viewport gets — the number
+    // is the viewport's, and what this test is about is that it does not
+    // change while the slider moves.
     assert_eq!(
         pass.detail_dispatches(),
-        2,
-        "a separable mask is two passes"
+        4,
+        "a reduced separable mask is four passes"
     );
     let pipelines = pass.cached_detail_pipelines();
 
@@ -501,17 +588,21 @@ fn dragging_the_slider_re_runs_the_detail_stage_and_nothing_else() {
         1,
         "the fused colour pass re-ran for a change it does not depend on"
     );
-    assert_eq!(pass.detail_dispatches(), 8);
+    // Four renders of a four-pass chain. The counter accumulates, so this is
+    // the claim that every one of those renders ran the detail stage and only
+    // the detail stage.
+    assert_eq!(pass.detail_dispatches(), 16);
     assert_eq!(
         pass.cached_detail_pipelines(),
         pipelines,
         "an amount is a uniform, not a shader"
     );
 
-    // Turning on the other control adds its own pair, and only its own pair.
+    // Turning on the other control adds its own pair, and only its own pair:
+    // 16, plus clarity's four again, plus texture's two.
     graph.set_param(TEXTURE, AMOUNT, 40.0);
     render(&mut pass, &graph, &source, 256);
-    assert_eq!(pass.detail_dispatches(), 12);
+    assert_eq!(pass.detail_dispatches(), 22);
     assert_eq!(pass.colour_dispatches(), 1);
 }
 
@@ -536,7 +627,11 @@ fn the_two_controls_stack_without_overwriting_each_other() {
     graph.set_param(TEXTURE, AMOUNT, 80.0);
     let mut pass = AdjustPass::new(&ctx);
     let both = row(&render(&mut pass, &graph, &source, SIZE), SIZE, SIZE / 2);
-    assert_eq!(pass.detail_dispatches(), 4);
+    // Clarity's four plus texture's two. Texture is never reduced — its band
+    // is a decade finer than clarity's, so a coarser grid could not hold its
+    // base — and the two operations keeping different pass counts here is that
+    // asymmetry showing through.
+    assert_eq!(pass.detail_dispatches(), 6);
 
     let edge = (SIZE / 2) as usize;
     // Both sides of the edge move the way local contrast moves them...