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/nxpmk66f18.ld

@@ -0,0 +1,38 @@
+
+/* Unused, but here to silence a linker warning. */
+ENTRY(Reset_Handler)
+
+/* define memory layout */
+MEMORY
+{
+    FLASH_TEXT (rx) : ORIGIN = 0x00000000, LENGTH = 1024K
+    RAM (rwx)       : ORIGIN = 0x1FFF0000, LENGTH = 256K
+}
+
+_stack_size = 2K;
+
+/* define output sections */
+SECTIONS
+{
+    /* Program code and read-only data goes to FLASH_TEXT. */
+    .text :
+    {
+        /* vector table MUST start at 0x0 */
+        . = 0;
+        KEEP(*(.isr_vector))
+
+        /* flash configuration MUST be at 0x400 */
+        . = 0x400;
+        KEEP(*(.flash_config))
+
+        /* everything else */
+        *(.text)
+        *(.text.*)
+        *(.rodata)
+        *(.rodata.*)
+        . = ALIGN(4);
+
+    } >FLASH_TEXT = 0xFF
+}
+
+INCLUDE "targets/arm.ld"