Say how far a detail chain reaches, for a tile's halo

radius() is the widest single pass. The chain's passes run in sequence,
so what a tile has to be grown by is their sum.

core/dr-pipeline/src/detail.rs

@@ -559,6 +559,26 @@ impl ComposedDetail {
             .max()
             .unwrap_or(0)
     }
+
+    /// TRACES: FR-DSP-2
+    /// How far the whole chain reads from the pixel it finally writes, in
+    /// render pixels: the halo a tile has to be grown by so that its interior
+    /// renders exactly as the untiled frame does.
+    ///
+    /// The **sum** of the passes' reaches, not the widest of them. The passes
+    /// run one after another, so a pixel of the last one depends on pixels of
+    /// the one before it `r` away, each of which depends on pixels a further
+    /// `r'` away. A separable blur's two halves each reach `r` along one axis
+    /// and the sum over-counts them by a factor of two; that is the price of a
+    /// bound that is always safe, and it is paid only by export tiles.
+    ///
+    /// One pixel per pass on top, for the reduced grids' bilinear taps.
+    pub fn reach(&self) -> u32 {
+        self.passes
+            .iter()
+            .map(|p| p.radius.saturating_mul(p.output_scale).saturating_add(1))
+            .fold(0u32, u32::saturating_add)
+    }
 }
 
 /// TRACES: FR-DEV-3 | FR-DSP-1