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/rp2040.ld

@@ -0,0 +1,36 @@
+MEMORY
+{
+    /* Reserve exactly 256 bytes at start of flash for second stage bootloader */
+    BOOT2_TEXT (rx) : ORIGIN = 0x10000000, LENGTH = 256
+    FLASH_TEXT (rx) : ORIGIN = 0x10000000 + 256, LENGTH = __flash_size - 256
+    RAM (rwx)       : ORIGIN = 0x20000000, LENGTH = 256k
+}
+
+_stack_size = 2K;
+
+SECTIONS
+{
+    /* Second stage bootloader is prepended to the image. It must be 256 bytes
+       and checksummed.  The gap to the checksum is zero-padded.
+    */
+    .boot2 : {
+        __boot2_start__ = .;
+        KEEP (*(.boot2));
+
+        /* Explicitly allocate space for CRC32 checksum at end of second stage
+           bootloader
+        */
+        . = __boot2_start__ + 256 - 4;
+        LONG(0)
+    } > BOOT2_TEXT = 0x0
+
+    /* The second stage will always enter the image at the start of .text.
+       The debugger will use the ELF entry point, which is the _entry_point
+       symbol if present, otherwise defaults to start of .text.
+       This can be used to transfer control back to the bootrom on debugger
+       launches only, to perform proper flash setup.
+    */
+}
+
+
+INCLUDE "targets/arm.ld"