FurShell: per-pixel z-order between fur objects (ZWrite On + coverage clip)

Transparent queue sorted fur draws per object, so overlapping fur
renderers swapped in front of each other frame to frame. FurForward
now writes depth; new Depth Sort Clip tunable discards low-coverage
fragments before they write depth (avoids halo punch-through).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Shaders/FurShell.shader

@@ -50,6 +50,12 @@ Shader "Custom/FurShell"
         _FurSoftness("Edge Softness (length fade)", Range(0.01, 1.0)) = 0.5
         // Per-strand anti-aliasing: soft strand silhouette + tip instead of hard alpha-clipped discs.
         _StrandSoftness("Strand Edge Softness", Range(0.0, 1.0)) = 0.3
+        // Fragments at/below this coverage are discarded so they don't WRITE DEPTH (ZWrite is on, see
+        // pass). Depth writes are what give two overlapping fur objects a correct per-pixel z-order
+        // (transparents otherwise sort per OBJECT and pop); the clip keeps faint edge pixels from
+        // occluding the other object with near-invisible fragments. Higher = harder strand edges but
+        // less edge halo between overlapping fur objects.
+        _DepthSortClip("Depth Sort Coverage Clip", Range(0.0, 1.0)) = 0.3
         // Opaque base shell. On = solid root (full-coverage prototype); Off = the root also clips to the
         // map so the painted body shows between strands (avatar integration).
         [ToggleUI] _SolidRoot("Solid Root", Float) = 1
@@ -97,15 +103,21 @@ Shader "Custom/FurShell"
 
             Cull Back
             Blend SrcAlpha OneMinusSrcAlpha
-            ZWrite Off
+            // ZWrite ON although this is a transparent blend: URP sorts transparents per OBJECT, so two
+            // overlapping fur objects would swap their whole-object z-order as the camera moves (visible
+            // popping where the fur balls overlap). Writing depth resolves the occlusion per PIXEL instead:
+            // whichever object draws first, the second one's hidden fragments fail ZTest. Fragments below
+            // _DepthSortClip coverage are discarded (see frag) so faint edge pixels don't occlude. Within
+            // one object the shells are submitted root->tip (= back-to-front on front faces), so its own
+            // blending still composites correctly.
+            ZWrite On
             ZTest LEqual
             // Depth bias toward the camera so the fur wins the depth test against the body. The fur
             // extrudes a CPU-baked mesh while the body uses GPU skinning, so their surfaces are not
             // bit-identical: where the body pokes a hair in front of a shell layer, that shell fragment
-            // fails ZTest (ZWrite Off => no depth write to win the tie) and the body flickers through as
-            // white speckles. Offset shifts the fragment's tested depth (respected even with ZWrite Off),
-            // slope-scaled + a constant unit, so the fur reliably passes without moving geometry (no
-            // visible root gap). Small enough not to punch through objects genuinely in front.
+            // fails ZTest and the body flickers through as white speckles. Offset shifts the fragment's
+            // depth, slope-scaled + a constant unit, so the fur reliably passes without moving geometry
+            // (no visible root gap). Small enough not to punch through objects genuinely in front.
             Offset -1, -1
 
             HLSLPROGRAM
@@ -141,6 +153,7 @@ Shader "Custom/FurShell"
                 float  _FurThreshold;
                 float  _FurSoftness;
                 float  _StrandSoftness;
+                float  _DepthSortClip;
                 float  _SolidRoot;
                 float  _WindStrength;
                 float  _WindSpeed;
@@ -344,7 +357,10 @@ Shader "Custom/FurShell"
                     alpha = (half)lenOpacity;
 
                 if (furLen <= _FurThreshold) alpha = 0.0h;
-                clip(alpha - 0.003h); // drop fully-transparent fragments (perf + avoids stray blend)
+                // Discard low-coverage fragments BEFORE they write depth (ZWrite is on for per-pixel
+                // sorting between overlapping fur objects): a near-invisible edge pixel must not occlude
+                // the fur behind it. The 0.003 floor keeps the old fully-transparent drop as a minimum.
+                clip(alpha - max((half)_DepthSortClip, 0.003h));
 
                 // --- shading (lightweight URP) ---
                 // base colour is the PAINTED ALBEDO (the camo under the fur), sampled per-texel so each