Add radial hero-cutout shader for third-person camera

Reveal the player when geometry sits between the orbit camera and the
character: a screen-space circular mask in a URP/Lit clone (HeroCutoutLit)
clips occluders in front of the player, depth-gated so only geometry nearer
than the hero is cut. The cutout lives outside UnityPerMaterial (SRP batcher
intact) and is a no-op when disarmed, so occluders look identical to URP/Lit.

A runtime driver (HeroCutout.cs on Main/GameManager) feeds the player's
screen position, radius, feather and depth into global shader properties and
only arms while the third-person orbit vcam is live; it disarms in
first-person, free-cam and the pause menu. Applied to the lobby walls via a
dedicated occluder material (floor left solid). Shipped but shelved for now.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scenes/Main.unity

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@@ -1864,6 +1865,24 @@ Transform:
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+--- !u!114 &1516578029
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