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/config.go

@@ -0,0 +1,43 @@
+package targets
+
+// Config represents a complete target configuration after inheritance resolution
+type Config struct {
+	// Target identification
+	Name string `json:"-"`
+
+	// LLVM configuration
+	LLVMTarget string `json:"llvm-target"`
+	CPU        string `json:"cpu"`
+	Features   string `json:"features"`
+
+	// Build configuration
+	BuildTags []string `json:"build-tags"`
+	GOOS      string   `json:"goos"`
+	GOARCH    string   `json:"goarch"`
+
+	// Compiler and linker configuration
+	Linker  string   `json:"linker"`
+	CFlags  []string `json:"cflags"`
+	LDFlags []string `json:"ldflags"`
+}
+
+// RawConfig represents the raw JSON configuration before inheritance resolution
+type RawConfig struct {
+	Inherits []string `json:"inherits"`
+	Config
+}
+
+// IsEmpty returns true if the config appears to be uninitialized
+func (c *Config) IsEmpty() bool {
+	return c.Name == "" && c.LLVMTarget == "" && c.GOOS == "" && c.GOARCH == ""
+}
+
+// HasInheritance returns true if this config inherits from other configs
+func (rc *RawConfig) HasInheritance() bool {
+	return len(rc.Inherits) > 0
+}
+
+// GetInherits returns the list of configs this config inherits from
+func (rc *RawConfig) GetInherits() []string {
+	return rc.Inherits
+}