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/avr.ld

@@ -0,0 +1,56 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0,                      LENGTH = __flash_size - _bootloader_size
+    RAM (xrw)       : ORIGIN = 0x800000 + __ram_start, LENGTH = __ram_size
+}
+
+ENTRY(main)
+
+SECTIONS
+{
+    .text :
+    {
+        KEEP(*(.vectors))
+        KEEP(*(.text.__vector_RESET))
+        KEEP(*(.text.main)) /* main must follow the reset handler */
+        *(.text)
+        *(.text.*)
+        *(.progmem)
+        *(.progmem.*)
+        . = ALIGN(16); /* needed with ld.lld for some reasoon */
+    }
+
+    .stack (NOLOAD) :
+    {
+        . += _stack_size;
+        _stack_top = .;
+    } >RAM
+
+    _sidata = LOADADDR(.data);
+
+    .data :
+    {
+        _sdata = .;        /* used by startup code */
+        *(.rodata)
+        *(.rodata.*)
+        *(.data)
+        *(.data*)
+        _edata = .;        /* used by startup code */
+    } >RAM AT>FLASH_TEXT
+
+    .bss :
+    {
+        _sbss = .;         /* used by startup code */
+        *(.bss)
+        *(.bss*)
+        *(COMMON)
+        _ebss = .;         /* used by startup code */
+    } >RAM
+}
+
+/* For the memory allocator. */
+_heap_start = _ebss;
+_heap_end = ORIGIN(RAM) + LENGTH(RAM);
+_globals_start = _sdata;
+_globals_end = _ebss;