Fix use of uninitialized FT in s_TSat Newton loop

The do while condition checked abs(FT) before FT was ever assigned.
Fortran .or. is not short-circuit, so abs(FT) was always evaluated
on the first iteration even though (ns == 0) guaranteed loop entry.

Restructure to do...exit: compute FT first, then check convergence.
Logically identical, no uninitialized reads, no intrinsics, safe on
all GPU device compilers (exit maps to a plain conditional branch).

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

Author: Spencer Bryngelson

src/common/m_phase_change.fpp

@@ -717,7 +717,7 @@ contains
 
             ! underrelaxation factor
             Om = 1.0e-3_wp
-            do while ((abs(FT) > ptgalpha_eps) .or. (ns == 0))
+            do
                 ! increasing counter
                 ns = ns + 1
 
@@ -738,6 +738,7 @@ contains
                 ! updating saturation temperature
                 TSat = TSat - Om*FT/dFdT
 
+                if (abs(FT) <= ptgalpha_eps) exit
             end do
 
         end if