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>

targets/esp32.json

@@ -0,0 +1,21 @@
+{
+	"inherits": ["xtensa"],
+	"cpu": "esp32",
+	"features": "+atomctl,+bool,+clamps,+coprocessor,+debug,+density,+dfpaccel,+div32,+exception,+fp,+highpriinterrupts,+interrupt,+loop,+mac16,+memctl,+minmax,+miscsr,+mul32,+mul32high,+nsa,+prid,+regprotect,+rvector,+s32c1i,+sext,+threadptr,+timerint,+windowed",
+	"build-tags": ["esp32", "esp"],
+	"scheduler": "tasks",
+	"serial": "uart",
+	"linker": "ld.lld",
+	"default-stack-size": 2048,
+	"rtlib": "compiler-rt",
+	"libc": "picolibc",
+	"linkerscript": "targets/esp32.ld",
+	"extra-files": [
+		"src/device/esp/esp32.S",
+		"src/internal/task/task_stack_esp32.S"
+	],
+	"binary-format": "esp32",
+	"flash-command": "esptool.py --chip=esp32 --port {port} write_flash 0x1000 {bin} -ff 80m -fm dout",
+	"emulator": "qemu-system-xtensa -machine esp32 -nographic -drive file={img},if=mtd,format=raw",
+	"gdb": ["xtensa-esp32-elf-gdb"]
+}