examples: First-person and third-person gsplat examples on Sunnyvale

[mvaligursky]

Adds two new Gaussian Splatting examples that demonstrate walking through a gsplat scene with physics, plus a new reusable third-person camera/character controller and a small enhancement to ShadowCatcher.

Examples (new):

• gaussian-splatting/first-person — First-person walkthrough of the Sunnyvale Heritage Park Museum gsplat with FPS-style movement, jump, mesh collision (driven by an invisible GLB collision proxy), and Ammo + Draco loaded up front.
• gaussian-splatting/third-person — Same scene with a third-person animated character (bitmoji), driven via a new ThirdPersonController script. Adds:
  • Orbiting camera with scroll-wheel zoom (smoothed), pitch/yaw clamps, wall-collision avoidance smoothing, configurable initial pitch and minimum camera height.
  • Animation state machine (idle / walk / jog / jump / dance) driven by the controller's speed / jump events plus a Q key to trigger the dance.
  • Env-atlas based IBL ambient lighting with the visible Skybox layer disabled (so the gsplat is the visible background).
  • Shadow catcher plane that follows the character on the ground via a downward raycast, with drawBucket: 0 so it renders after the gsplat.
  • Inverted-look-Y example wiring.
• Scene assets streamed from S3 (same pattern as lod-streaming), with CC BY 4.0 attribution to zeitgeistarchivescans (https://superspl.at/scene/d5d397aa) in the example headers.
• Thumbnails added for both examples (gaussian-splatting_first-person_*.webp, gaussian-splatting_third-person_*.webp).

New reusable script: scripts/esm/third-person-controller.mjs

Modeled directly after first-person-controller.mjs (same input pipeline: KeyboardMouseSource + DualGestureSource + GamepadSource, same grounded-raycast pattern). Public @attribute API:

• Entity refs: camera, characterModel
• Movement: lookSens, invertLookY, speedGround, speedAir, sprintMult, velocityDampingGround, velocityDampingAir, jumpForce
• Orbit camera: initialPitch, pitchMin, pitchMax, cameraDistance, cameraDistanceMin, cameraDistanceMax, cameraHeight, cameraMinHeightAboveCharacter, cameraMinDistance, cameraCollisionPadding, cameraPositionDamping, cameraLookAtDamping, cameraCollisionDamping, zoomSpeed, zoomDamping
• Character model rotation: modelTurnSmoothing, modelYawOffset
• Anim hook thresholds: walkSpeedThreshold, jogSpeedThreshold
• Mobile/gamepad dead-zones and joystick UI event name (same as FPS controller)

Fires script events speed (0/1/2 idle/walk/jog when the bucket changes) and jump so consumers can drive an anim state graph without coupling.

ShadowCatcher enhancement: scripts/esm/shadow-catcher.mjs

• New @attribute drawBucket (default 250 — preserves existing behaviour). Lets consumers control where the catcher sits in the transparent render order without poking mesh instances. Useful when the catcher needs to render after a transparent object like a gsplat ground (set to 0).
• The mesh-instance assignment is now applied every frame in update() so runtime changes to drawBucket (and the existing scale) take effect.