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/rp2350.ld

@@ -0,0 +1,23 @@
+/* See Rust for a more complete reference: https://github.com/rp-rs/rp-hal/blob/main/rp235x-hal-examples/memory.x */
+MEMORY
+{
+   /* 2MiB safe default.  */
+    FLASH : ORIGIN = 0x10000000, LENGTH = __flash_size
+    /* RAM consists of 8 banks, SRAM0..SRAM7 with striped mapping. */
+    SRAM : ORIGIN = 0x20000000, LENGTH = 512k
+    /* Banks 8 and 9 use direct mapping which can be
+       specailized for applications where predictable access time is beneficial.
+       i.e: Separate stacks for core0 and core1. */
+    SRAM4 : ORIGIN = 0x20080000, LENGTH = 4k
+    SRAM5 : ORIGIN = 0x20081000, LENGTH = 4k
+    FLASH_TEXT (rx) : ORIGIN = 0x10000000, LENGTH = __flash_size
+    RAM (rwx)       : ORIGIN = 0x20000000, LENGTH = 512k
+}
+
+_stack_size = 2K;
+
+SECTIONS
+{
+}
+
+INCLUDE "targets/arm.ld"