fix(webgl): Add ClearTarget and always rebuild in test script

1. gal_webgl_test.cpp: Add ClearTarget(TARGET_NONCACHED) before ClearScreen()
   - Matches native test harness behavior
   - Fixes accumulated content from previous scenarios
   - arc-segments now 10% different (was 86% due to content accumulation)

2. test-gal-regression.sh: Remove "skip if already built" check
   - Always rebuild to pick up code changes
   - Single script is the source of truth for full test cycle

Current results:
- 3 scenarios MATCH (<1%): basic-lines, arcs, transforms
- Many scenarios close (~10%): arc-segments, circles, hole-walls, etc.
- Some features not implemented (97-100%): glyphs, text-attrs, bitmap

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/gal-regression/wasm/gal_webgl_test.cpp

@@ -57,7 +57,10 @@ void renderCurrentScenario() {
     // Set target to non-cached for immediate rendering
     g_gal->SetTarget(KIGFX::TARGET_NONCACHED);
 
-    // Clear the screen
+    // Clear the target buffer first (clears the FBO where content is rendered)
+    g_gal->ClearTarget(KIGFX::TARGET_NONCACHED);
+
+    // Clear the screen (clears the direct rendering buffer)
     g_gal->ClearScreen();
 
     // Render the scenario