wip(assembler): rounds L5+L6 -- Linux far-terrain captures and screen-space gate (world unchanged)

Linux offscreen capture (EGL/GLES pbuffer; llvmpipe in the sandbox) and an opt-in screen-space gate
for LOD>=3 (normal distance projected at each LOD's closest display distance, 1 px at 1440p =
0.61 mrad). The gate rejects the S13 octave-drop and stride-2 controls, fault injections, and a
vertical-vs-screen control. Finding: today's far meshes differ from the field by up to 92 px, and
LOD swaps snap mesa rims by tens of px (p50 0 px), a silhouette effect of plain LOD switching. The
octave cutoff was dropped: nothing qualified, and octaves are a small share of far time. Off-path
cost: reviewer interleaved W12 within noise vs L4 (+0.6% median). Hashes = L4.
Largest far lever measured: column work outside noise, 34% of far worker time (duplicate XY across tiles).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ED33VnqepD4YY6gDAAA832

Standalone/BuildLinux.sh

@@ -33,6 +33,8 @@ if [[ "$FP_CONTRACT" != "default" && "$FP_CONTRACT" != "off" && "$FP_CONTRACT" !
 fi
 command -v "$COMPILER" >/dev/null || { printf 'Compiler not found: %s\n' "$COMPILER" >&2; exit 2; }
 command -v ar >/dev/null || { printf 'ar not found\n' >&2; exit 2; }
+command -v pkg-config >/dev/null || { printf 'pkg-config not found\n' >&2; exit 2; }
+pkg-config --exists egl glesv2 libpng || { printf 'EGL, GLESv2, and libpng development files are required\n' >&2; exit 2; }
 
 BUILD_DIR="$ROOT/Build/linux-$VARIANT"
 BIN_DIR="$ROOT/bin/linux"
@@ -117,5 +119,10 @@ CORE_LIBRARY="$BUILD_DIR/libVoxelForgeCore.a"
 "$(command -v ar)" rcs "$CORE_LIBRARY" "${CORE_OBJECTS[@]}"
 OUTPUT="$BIN_DIR/VoxelForgeCoreHeadless-$VARIANT"
 "$COMPILER" -pthread "${LEGACY_OBJECTS[@]}" "$CORE_LIBRARY" -o "$OUTPUT"
+CAPTURE_OUTPUT="$BIN_DIR/VoxelForgeOffscreenCapture-$VARIANT"
+"$COMPILER" -std=c++20 -O2 -fno-fast-math -pthread \
+    "$ROOT/Source/LinuxOffscreenCapture.cpp" -o "$CAPTURE_OUTPUT" \
+    $(pkg-config --cflags --libs egl glesv2 libpng)
 printf 'Built Core library: %s\nBuilt headless benchmark: %s\n' "$CORE_LIBRARY" "$OUTPUT"
+printf 'Built EGL offscreen capture: %s\n' "$CAPTURE_OUTPUT"
 printf 'Compiler: %s | variant: %s | fp-contract: %s | jobs: %s\n' "$COMPILER" "$VARIANT" "$FP_CONTRACT" "$JOBS"