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/main.go

@@ -0,0 +1,67 @@
+package main
+
+import (
+	"github.com/goplus/lib/c"
+)
+
+// This demo shows how to use //export with different symbol names on embedded targets.
+//
+// On embedded targets, you can export Go functions with different C symbol names.
+// This is useful for hardware interrupt handlers that require specific names.
+
+// Standard Go export - same name
+//
+//export HelloWorld
+func HelloWorld() {
+	c.Printf(c.Str("Hello from "))
+	c.Printf(c.Str("HelloWorld\n"))
+}
+
+// Embedded target export - different name
+// Go function name: interruptLPSPI2
+// Exported C symbol: LPSPI2_IRQHandler
+//
+//export LPSPI2_IRQHandler
+func interruptLPSPI2() {
+	c.Printf(c.Str("LPSPI2 interrupt "))
+	c.Printf(c.Str("handler called\n"))
+}
+
+// Embedded target export - different name
+// Go function name: systemTickHandler
+// Exported C symbol: SysTick_Handler
+//
+//export SysTick_Handler
+func systemTickHandler() {
+	c.Printf(c.Str("SysTick "))
+	c.Printf(c.Str("handler called\n"))
+}
+
+// Embedded target export - different name
+// Go function name: Add
+// Exported C symbol: AddFunc
+//
+//export AddFunc
+func Add(a, b int) int {
+	result := a + b
+	c.Printf(c.Str("AddFunc(%d, %d) = %d\n"), a, b, result)
+	return result
+}
+
+func main() {
+	c.Printf(c.Str("=== Export Demo ===\n\n"))
+
+	// Call exported functions directly from Go
+	c.Printf(c.Str("Calling HelloWorld:\n"))
+	HelloWorld()
+
+	c.Printf(c.Str("\nSimulating hardware interrupts:\n"))
+	interruptLPSPI2()
+	systemTickHandler()
+
+	c.Printf(c.Str("\nTesting function with return value:\n"))
+	result := Add(10, 20)
+	c.Printf(c.Str("Result: %d\n"), result)
+
+	c.Printf(c.Str("\n=== Demo Complete ===\n"))
+}