feat: implement target configuration system for issue #1176

Add comprehensive target configuration parsing and inheritance system:

- Create internal/targets package with config structures
- Support JSON configuration loading with inheritance resolution
- Implement multi-level inheritance (e.g., rp2040 → cortex-m0plus → cortex-m)
- Add 206 target configurations from TinyGo for embedded platforms
- Support core fields: name, llvm-target, cpu, features, build-tags, goos, goarch, cflags, ldflags
- Provide high-level resolver interface for target lookup
- Include comprehensive unit tests with 100% target parsing coverage

This foundation enables future -target parameter support for cross-compilation
to diverse embedded platforms beyond current GOOS/GOARCH limitations.

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

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

internal/targets/example_test.go

@@ -0,0 +1,73 @@
+package targets_test
+
+import (
+	"fmt"
+	"log"
+	"sort"
+
+	"github.com/goplus/llgo/internal/targets"
+)
+
+func ExampleResolver_Resolve() {
+	resolver := targets.NewDefaultResolver()
+
+	// Resolve a specific target
+	config, err := resolver.Resolve("rp2040")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("Target: %s\n", config.Name)
+	fmt.Printf("LLVM Target: %s\n", config.LLVMTarget)
+	fmt.Printf("CPU: %s\n", config.CPU)
+	fmt.Printf("GOOS: %s\n", config.GOOS)
+	fmt.Printf("GOARCH: %s\n", config.GOARCH)
+	if len(config.BuildTags) > 0 {
+		fmt.Printf("Build Tags: %v\n", config.BuildTags)
+	}
+	if len(config.CFlags) > 0 {
+		fmt.Printf("C Flags: %v\n", config.CFlags)
+	}
+	if len(config.LDFlags) > 0 {
+		fmt.Printf("LD Flags: %v\n", config.LDFlags)
+	}
+}
+
+func ExampleResolver_ListAvailableTargets() {
+	resolver := targets.NewDefaultResolver()
+
+	targets, err := resolver.ListAvailableTargets()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Show first 10 targets
+	sort.Strings(targets)
+	fmt.Printf("Available targets (first 10 of %d):\n", len(targets))
+	for i, target := range targets[:10] {
+		fmt.Printf("%d. %s\n", i+1, target)
+	}
+}
+
+func ExampleResolver_ResolveAll() {
+	resolver := targets.NewDefaultResolver()
+
+	configs, err := resolver.ResolveAll()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Count targets by GOOS
+	goosCounts := make(map[string]int)
+	for _, config := range configs {
+		if config.GOOS != "" {
+			goosCounts[config.GOOS]++
+		}
+	}
+
+	fmt.Printf("Loaded %d target configurations\n", len(configs))
+	fmt.Printf("GOOS distribution:\n")
+	for goos, count := range goosCounts {
+		fmt.Printf("  %s: %d targets\n", goos, count)
+	}
+}