Fix arithmetic expression line splits in collision physics

Fixed 2 instances of unnecessarily split arithmetic expressions in
collision handling code. Expressions are now more readable on single lines
while staying under 120 characters.

Following Catalyst.jl PR #1306 guidelines.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: claude

test/downstream/community_callback_tests.jl

@@ -155,12 +155,8 @@ function affect!(integrator, idx)
                 x₂ = u.nodes[l][1:2]
                 v₂ = u.nodes[l][3:4]
                 # https://stackoverflow.com/a/35212639
-                v₁ = (v₁ -
-                      2 / (1 + 1) *
-                      (dot(v₁ - v₂, x₁ - x₂) / sum(abs2, x₁ - x₂) * (x₁ - x₂)))
-                v₂ = -(v₂ -
-                       2 / (1 + 1) *
-                       (dot(v₂ - v₁, x₂ - x₁) / sum(abs2, x₂ - x₁) * (x₂ - x₁)))
+                v₁ = (v₁ - 2 / (1 + 1) * (dot(v₁ - v₂, x₁ - x₂) / sum(abs2, x₁ - x₂) * (x₁ - x₂)))
+                v₂ = -(v₂ - 2 / (1 + 1) * (dot(v₂ - v₁, x₂ - x₁) / sum(abs2, x₂ - x₁) * (x₂ - x₁)))
 
                 println("Collision handled.")