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/pico-boot-stage2.S

@@ -0,0 +1,17 @@
+// Raspberry Pi Pico Stage 2 Bootloader
+
+//
+// This file defines the parameters specific to the flash-chip found
+// on the official Pico boards.  The generic implementation is in
+// rp2040-boot-stage2.S
+//
+
+#define BOARD_PICO_FLASH_SPI_CLKDIV   2
+#define BOARD_CMD_READ                0xeb
+#define BOARD_QUAD_OK                 1
+#define BOARD_QUAD_ENABLE_STATUS_BYTE 2
+#define BOARD_QUAD_ENABLE_BIT_MASK    2
+#define BOARD_SPLIT_STATUS_WRITE      0
+#define BOARD_WAIT_CYCLES             4
+
+#include "rp2040-boot-stage2.S"