Fix plunger pullback, kinematic motion, and mesh tip

[freezy]

Summary

• Fix plunger pullback/fire command behavior and analog mechanical plunger handling
• Keep plunger collider and generated Unity meshes in local Z so transformed plungers behave correctly
• Update kinematic plunger displacement/collision handling
• Close the generated rod tip mesh and harden spring mesh generation for empty spring data
• Add regression coverage for plunger command behavior, transformed assumptions, local mesh/collider Z, kinematic motion, and mesh generation

Testing

• dotnet test VisualPinball.Engine.Test\VisualPinball.Engine.Test.csproj -c Release --filter FullyQualifiedName~Plunger -v:minimal

Note: dotnet build VisualPinball.Unity\VisualPinball.Unity.Test\VisualPinball.Unity.Test.csproj -c Release does not complete in this shell because Unity/package-only references such as NativeTrees, Newtonsoft, GLTFast, and Unity collections types are unavailable outside the Unity project environment.

[greptile-apps]

Greptile Summary

This PR updates plunger behavior and geometry handling. The main changes are:

• Fixed pullback, fire, retract, and analog mechanical plunger state handling.
• Kept plunger collider and generated mesh data in local Z space.
• Updated kinematic plunger displacement and collision velocity handling.
• Closed the generated rod tip mesh and guarded empty spring mesh generation.
• Added tests for plunger commands, physics, local mesh/collider Z, and mesh generation.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
VisualPinball.Engine/VPT/Plunger/PlungerMeshGenerator.cs	Adds local mesh generation, closes the rod tip, and handles empty spring meshes.
VisualPinball.Unity/VisualPinball.Unity/VPT/Plunger/PlungerCollider.cs	Moves plunger collider bounds to local Z and includes surface velocity in collision response.
VisualPinball.Unity/VisualPinball.Unity/Game/PhysicsCycle.cs	Routes kinematic plunger displacement through the kinematic collider collection.
VisualPinball.Unity/VisualPinball.Unity/Game/PhysicsStaticCollision.cs	Passes kinematic surface velocity into plunger collision handling.
VisualPinball.Unity/VisualPinball.Unity/VPT/Plunger/PlungerVelocityPhysics.cs	Maps analog mechanical plunger input into the valid rest-to-full-pull range.
VisualPinball.Unity/VisualPinball.Unity/VPT/Plunger/PlungerCommands.cs	Resets pullback and retract motion state more explicitly.
VisualPinball.Unity/VisualPinball.Unity/Game/Player.cs	Registers analog plunger callbacks for performed and canceled input phases.
VisualPinball.Unity/VisualPinball.Unity/Input/InputManager.cs	Changes the default analog plunger binding to a processed value-axis input.

_{Reviews (1): Last reviewed commit: "plunger: close rod tip mesh" | Re-trigger Greptile}