test: add comprehensive tests and CI for export feature

Add extensive test coverage, demo program, and CI integration for
//export with different names feature:

Unit Tests (cl/builtin_test.go):
- TestHandleExportDiffName: core functionality with 4 scenarios
  * Different names with enableExportRename
  * Same names with enableExportRename
  * Different names with spaces in export directive
  * Matching names without enableExportRename
- TestInitLinknameByDocExportDiffNames: flag behavior verification
  * Export with different names when enabled
  * Export with same name when enabled
  * Normal linkname directives
- TestInitLinkExportDiffNames: edge case handling
  * Symbol not found in decl packages (silent handling)

Demo (_demo/embed/export/):
- Example program demonstrating various export patterns
- Verification script testing both embedded and non-embedded targets
- Documents expected behavior and error cases

CI Integration (.github/workflows/llgo.yml):
- Add export demo to embedded target tests
- Ensure feature works correctly across platforms
- Catch regressions in future changes

The tests verify:
✓ Different names work with -target flag
✓ Same names work in all cases
✓ Different names fail without -target flag
✓ Proper error messages for invalid exports
✓ Silent handling for decl packages

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

_demo/embed/export/verify_export.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+
+set -e
+
+echo "Building for embedded target..."
+
+# Build for embedded target as executable
+# Use llgo directly instead of llgo.sh to avoid go.mod version check
+llgo build -o test-verify --target=esp32 .
+
+echo "Checking exported symbols..."
+
+# Get exported symbols
+exported_symbols=$(nm -gU ./test-verify.elf | grep -E "(HelloWorld|LPSPI2_IRQHandler|SysTick_Handler|AddFunc)" | awk '{print $NF}')
+
+echo ""
+echo "Exported symbols:"
+echo "$exported_symbols" | awk '{print "  " $0}'
+echo ""
+
+# Check expected symbols
+expected=("HelloWorld" "LPSPI2_IRQHandler" "SysTick_Handler" "AddFunc")
+missing=""
+
+for symbol in "${expected[@]}"; do
+    if ! echo "$exported_symbols" | grep -q "^$symbol$"; then
+        missing="$missing $symbol"
+    fi
+done
+
+if [ -n "$missing" ]; then
+    echo "❌ Missing symbols:$missing"
+    exit 1
+fi
+
+echo "✅ Symbol name mapping verification:"
+echo "  HelloWorld           -> HelloWorld"
+echo "  interruptLPSPI2      -> LPSPI2_IRQHandler"
+echo "  systemTickHandler    -> SysTick_Handler"
+echo "  Add                  -> AddFunc"
+echo ""
+echo "🎉 All export symbols verified successfully!"
+echo ""
+
+echo "Testing that non-embedded target rejects different export names..."
+# Build without --target should fail with panic
+if llgo build -o test-notarget . 2>&1 | grep -q 'export comment has wrong name "LPSPI2_IRQHandler"'; then
+    echo "✅ Correctly rejected different export name on non-embedded target"
+else
+    echo "❌ Should have panicked with 'export comment has wrong name' error"
+    exit 1
+fi
+echo ""
+echo "Note: Different symbol names are only supported on embedded targets."