art/brlcadplugin: fix thread-safety in brlcad_hit() ray result passing

[rupeshca007]

Summary

Fixes a thread-safety bug in the Appleseed renderer's ray intersection bridge (src/art/brlcadplugin.cpp).

Related to GSoC 2026 issue: opencax/GSoC#108 (Stabilize Appleseed Rendering)

Problem

brlcad_ray_info was a global thread_local struct used to pass hit results from the brlcad_hit() callback back to intersect():

thread_local struct BRLCAD_to_ASR brlcad_ray_info;  // global
// brlcad_hit writes to it:
brlcad_ray_info.distance = hitp->hit_dist;
// intersect() reads from it:
result.m_distance = brlcad_ray_info.distance;

This is unsafe: if Appleseed's multi-threaded renderer dispatches brlcad_hit() on a worker thread different from the thread calling intersect(), the thread_local read returns a different (stale/uninitialized) copy, causing silent data corruption or crashes.

Fix

Allocate a BRLCAD_to_ASR struct on the stack in intersect() and pass its address through app.a_uptr. brlcad_hit() now writes directly into the caller's local struct:

// intersect():
struct BRLCAD_to_ASR local_ray_info = {};
app.a_uptr = (void*)&local_ray_info;
rt_shootray(&app);
result.m_distance = local_ray_info.distance;

// brlcad_hit():
struct BRLCAD_to_ASR* ray_info = (struct BRLCAD_to_ASR*)ap->a_uptr;
ray_info->distance = hitp->hit_dist;

This eliminates all shared global state in the intersection hot path. Each intersect() call is now fully self-contained and thread-safe.

Also removed

• Dead app.a_uptr = name assignment (was overwritten immediately after)
• • Commented-out UV/normal-flip calculations (never enabled)