Restore standard board collision group when entering riding

crates/skate-game/src/physics/ground_runtime/entry.rs

@@ -65,6 +65,10 @@ impl GroundState {
         //source-owned wrapper flag. Authored board volumes stay enabled.
         *targets.board_flags_8384 &= 0x7f;
         board.bodies_mut()[6].inertia.angular_drag = self.entry_settings.deck_angular_drag;
+        //82C090C0 restores assembly+24 and every part+92 to collision group4.
+        //Remount releases the carried board into group7; retaining that group
+        //allows the riding skeleton (group5) to collide with its own board.
+        board.set_collision_group(4);
         for body in board.bodies_mut() {
             body.state_flags = 4;
         }

docs/remount-collision-group.md

@@ -0,0 +1,36 @@
+# Riding collision group restoration
+
+## Original behavior
+
+Reference: Skate 3 TU3 image `default_82000000_011B0000.bin`, base
+`0x82000000`, SHA-256
+`f4aa113eb541bfba03dbc108cf5ab43f58c965b20fa3b82f9c40938a0ad841c4`.
+
+Observed in the generated PPC source: Ground entry `0x82D37538` calls
+SetStandard `0x82C03AF8`, which calls `0x82C091F8` (standard deck angular
+drag), `0x82C090C0` (standard collision groups/materials), and `0x82C06ED8`
+(volume enable flags). In `0x82C090C0`, `li r8,4` supplies the value written
+to assembly+24 and each 96-byte part's +92 collision group.
+
+The imported Ground entry already restored deck drag, but omitted the group
+assignment. Offboard board release assigns group 7. Riding skeleton mode 6
+restores rider parts to group 5. The single-player table from `0x82765EF0`
+culls group 4 against 5, but allows group 7 against 5. Consequently a remount
+could leave riding enabled while retaining board/rider self-collisions.
+
+## Evidence and interpretation
+
+User reported the persistent skating fault begins after getting back on the
+board. Capture `logs/game-20260908-024133.stderr.log` contains nonzero deck
+split-position/orientation contact reactions (for example solver tick 2178),
+while the animation hook impulse is zero and the offboard hand drives are no
+longer present. The preceding `023133` capture shows continued roll and
+three-wheel contact despite neutral steering. These observations support
+unwanted board/rider contacts as the cause; the captures did not directly
+record the board collision-group value.
+
+Ground entry now restores group 4 at the native SetStandard point. This is
+a missing source assignment, not a friction, steering, or balance adjustment.
+The source-level omission is confirmed; symptom resolution awaits the user's
+remount gameplay check. Compilation passed with `cargo build -p skate-game
+--locked`; no automated tests or gameplay tests were run.