Let an operation read the pixel next to it, and settle where sharpening belongs

The fused pass hands a fragment a colour and no coordinate. That is what buys
one dispatch for a whole edit, and it is also a wall: sharpening, noise
reduction, clarity, texture, dehaze and spot removal are each defined by what
the neighbours are doing, and FR-DEV-3 and FR-DEV-8 ask for all six. None of
them could be written at any price.

So there is now a detail stage. An operation implements `Operation` for its
parameters exactly as before — the panel, the sidecar, the history and the
presets all work unchanged — and additionally returns `Affects::Detail` and a
`DetailStage` yielding one pass per dispatch. `Affects` grows the third variant
`docs/requirements.md:250` designed and nothing had cut.

Where the stage sits is a colour-science decision, not an arrangement of
convenience. It runs after every point operation and every mask layer, so an
amount chosen against a tone curve survives the curve moving; in linear sRGB
after the camera matrix, because camera RGB has no luminance to sharpen
against; and before the output transform and the clip, because FR-DEV-2 allows
one quantisation and a highlight clipped before a convolution grows a dark
ring. The fused pass therefore ends one of two ways, and when a detail stage
follows it hands on unclipped f16 and the last detail pass encodes.

At render resolution rather than on the source, which is the whole of FR-DSP-1:
a pass before the framing prologue would cost 24 MP to draw a 2 MP preview.
`RenderScale` is what makes that survivable — a radius is stored as a fraction
of the frame's shorter edge, exactly as a mask feather already is, or as a
count of source pixels, and converted per render. It also reports when a radius
is smaller than a proxy pixel rather than drawing a plausible lie; zooming to
1:1 makes the preview exact with no second path.

`Invalidation` gives FR-DEV-3d something to mean. Moving a detail parameter
leaves the colour key alone, so `AdjustPass` keeps the linear intermediate and
skips the fused dispatch: dragging a sharpening slider costs a convolution.
Moving exposure does re-run the detail passes, because they read what the
colour pass wrote, and there is no arrangement of keys that avoids it while
keeping sharpening after tone.

Validated by a separable box blur that is not a develop operation, behind the
`detail-probe` feature and absent from a shipping build. An abstraction with no
consumer is a guess; a box blur's answer is known in closed form, so the tests
assert every byte of the ramp rather than that the edge got softer.

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

core/dr-gpu/Cargo.toml

@@ -25,6 +25,12 @@ pollster.workspace = true
 
 [dev-dependencies]
 env_logger.workspace = true
+# The detail stage's test consumer — a box blur that is not a develop operation
+# and never reaches the panel. An abstraction with no consumers is a guess, and
+# this is the one that proves the neighbourhood passes compile, ping-pong,
+# encode once, and scale between a proxy and an export. A dev-dependency, so a
+# shipping `dr-gpu` does not carry it.
+dr-pipeline = { workspace = true, features = ["detail-probe"] }
 # The local-adjustment example needs the model, which the library half of this
 # crate deliberately does not: `dr-gpu` holds the shaders, and the inference
 # runtime belongs to whoever is asking a question about the picture.