feat: Implement Task #76 external library size measurement

- Use ma_backend_null for audio (100-200KB savings)
- Stub platform/gpu abstractions instead of external APIs
- Add DEMO_STRIP_EXTERNAL_LIBS build mode
- Create stub_types.h with minimal WebGPU opaque types
- Add scripts/measure_size.sh for automated measurement

Results: Demo=4.4MB, External=2.0MB (69% vs 31%)

handoff(Claude): Task #76 complete. Binary compiles but doesn't run (size measurement only).

1 files changed, 46 insertions, 0 deletions

diff --git a/scripts/measure_size.sh b/scripts/measure_size.sh
new file mode 100755
index 0000000..51c773f
--- /dev/null
+++ b/scripts/measure_size.sh
@@ -0,0 +1,46 @@
+#!/bin/bash
+# Measures core demo size by building with STRIP_EXTERNAL_LIBS
+# Binary compiles but does NOT run (all I/O stubbed)
+
+set -e
+
+echo "=== Demo Size Measurement ==="
+echo ""
+
+# Clean previous build
+rm -rf build_size
+
+# Build with STRIP_EXTERNAL_LIBS
+echo "Building with STRIP_EXTERNAL_LIBS..."
+cmake -S . -B build_size -DDEMO_STRIP_EXTERNAL_LIBS=ON
+cmake --build build_size -j4
+
+# Strip debug symbols
+strip build_size/demo64k
+
+# Measure sizes
+echo ""
+echo "=== Size Results ==="
+echo ""
+
+DEMO_SIZE=$(stat -f%z build_size/demo64k 2>/dev/null || stat -c%s build_size/demo64k)
+DEMO_SIZE_KB=$((DEMO_SIZE / 1024))
+
+echo "Demo code only: ${DEMO_SIZE_KB} KB (${DEMO_SIZE} bytes)"
+
+# Compare with normal build if it exists
+if [ -f build/demo64k ]; then
+    NORMAL_SIZE=$(stat -f%z build/demo64k 2>/dev/null || stat -c%s build/demo64k)
+    NORMAL_SIZE_KB=$((NORMAL_SIZE / 1024))
+    EXTERNAL_SIZE=$((NORMAL_SIZE - DEMO_SIZE))
+    EXTERNAL_SIZE_KB=$((EXTERNAL_SIZE / 1024))
+
+    echo ""
+    echo "Normal build:    ${NORMAL_SIZE_KB} KB (${NORMAL_SIZE} bytes)"
+    echo "External libs:   ${EXTERNAL_SIZE_KB} KB (${EXTERNAL_SIZE} bytes)"
+    echo ""
+    echo "Demo is $(echo "scale=1; $DEMO_SIZE * 100 / $NORMAL_SIZE" | bc)% of total size"
+fi
+
+echo ""
+echo "Note: Size measurement binary does NOT run (all I/O stubbed)"