Add missing A, B, C to THINC GPU private clause

These scalars are written per-thread inside the parallel loop
(lines 278-280) but were missing from the private list, causing
a GPU data race on all backends.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_muscl.fpp

@@ -253,7 +253,7 @@ contains
         #:for MUSCL_DIR, XYZ in [(1, 'x'), (2, 'y'), (3, 'z')]
             if (muscl_dir == ${MUSCL_DIR}$) then
 
-                $:GPU_PARALLEL_LOOP(collapse=3,private='[j,k,l,aCL,aC,aCR,aTHINC,moncon,sign,qmin,qmax,rho_b,rho_e]')
+                $:GPU_PARALLEL_LOOP(collapse=3,private='[j,k,l,aCL,aC,aCR,aTHINC,moncon,sign,qmin,qmax,A,B,C,rho_b,rho_e]')
                 do l = is3_muscl%beg, is3_muscl%end
                     do k = is2_muscl%beg, is2_muscl%end
                         do j = is1_muscl%beg, is1_muscl%end