wip(assembler): round L2 -- the cross-platform hash check can fail; FP contraction changes the world

Clang -ffp-contract=on/fast and GCC's default (fast) all change the density hash on the L1 view;
contraction off reproduces L1 (= Windows S13) at W1/W12. Seed sweep: 48 FloatingIslands seeds and
14 effective SurfaceWorld seeds (34 of 48 fell on a seed-independent cube) match across Clang/GCC
and W1/W12. Standalone gains --seed/--recipe/--render-distance-chunks (viewer + headless) and a
Windows replay script. Linux standalone workers get 32 MiB pthread stacks (TLS is 17.3 MiB/thread),
so no external stack limit is needed. No Core or world-output change; MSVC compile unverified.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Standalone/BuildLinux.sh

@@ -27,8 +27,8 @@ if [[ ! "$JOBS" =~ ^[0-9]+$ ]] || ((JOBS < 1 || JOBS > 12)); then
     printf 'jobs must be from 1 to 12\n' >&2
     exit 2
 fi
-if [[ "$FP_CONTRACT" != "off" && "$FP_CONTRACT" != "on" && "$FP_CONTRACT" != "fast" ]]; then
-    printf 'fp-contract must be off, on, or fast\n' >&2
+if [[ "$FP_CONTRACT" != "default" && "$FP_CONTRACT" != "off" && "$FP_CONTRACT" != "on" && "$FP_CONTRACT" != "fast" ]]; then
+    printf 'fp-contract must be default, off, on, or fast\n' >&2
     exit 2
 fi
 command -v "$COMPILER" >/dev/null || { printf 'Compiler not found: %s\n' "$COMPILER" >&2; exit 2; }
@@ -47,9 +47,12 @@ INCLUDES=(
     "-I$ROOT/Source/VoxelForge/Private"
 )
 FLAGS=(
-    -std=c++20 -O2 -mavx2 -mfma "-ffp-contract=$FP_CONTRACT" -fno-fast-math
+    -std=c++20 -O2 -mavx2 -mfma -fno-fast-math
     -fexceptions -pthread "${INCLUDES[@]}"
 )
+if [[ "$FP_CONTRACT" != "default" ]]; then
+    FLAGS+=("-ffp-contract=$FP_CONTRACT")
+fi
 if [[ "$($COMPILER --version | head -n 1)" == *clang* ]]; then
     FLAGS+=(-Wno-c++11-narrowing)
 else