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/gameboy-advance.s

@@ -0,0 +1,77 @@
+.section    .init
+.global     _start
+.align
+.arm
+
+_start:
+    b       start_vector
+
+    // ROM header
+    .byte 0x24,0xff,0xae,0x51,0x69,0x9a,0xa2,0x21,0x3d,0x84,0x82,0x0a,0x84,0xe4,0x09,0xad
+    .byte 0x11,0x24,0x8b,0x98,0xc0,0x81,0x7f,0x21,0xa3,0x52,0xbe,0x19,0x93,0x09,0xce,0x20
+    .byte 0x10,0x46,0x4a,0x4a,0xf8,0x27,0x31,0xec,0x58,0xc7,0xe8,0x33,0x82,0xe3,0xce,0xbf
+    .byte 0x85,0xf4,0xdf,0x94,0xce,0x4b,0x09,0xc1,0x94,0x56,0x8a,0xc0,0x13,0x72,0xa7,0xfc
+    .byte 0x9f,0x84,0x4d,0x73,0xa3,0xca,0x9a,0x61,0x58,0x97,0xa3,0x27,0xfc,0x03,0x98,0x76
+    .byte 0x23,0x1d,0xc7,0x61,0x03,0x04,0xae,0x56,0xbf,0x38,0x84,0x00,0x40,0xa7,0x0e,0xfd
+    .byte 0xff,0x52,0xfe,0x03,0x6f,0x95,0x30,0xf1,0x97,0xfb,0xc0,0x85,0x60,0xd6,0x80,0x25
+    .byte 0xa9,0x63,0xbe,0x03,0x01,0x4e,0x38,0xe2,0xf9,0xa2,0x34,0xff,0xbb,0x3e,0x03,0x44
+    .byte 0x78,0x00,0x90,0xcb,0x88,0x11,0x3a,0x94,0x65,0xc0,0x7c,0x63,0x87,0xf0,0x3c,0xaf
+    .byte 0xd6,0x25,0xe4,0x8b,0x38,0x0a,0xac,0x72,0x21,0xd4,0xf8,0x07
+
+    // Game title
+    .byte 0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00
+
+    // Game code
+    .byte 0x00,0x00,0x00,0x00
+
+    // Maker code
+    .byte 0x00,0x00
+
+    // Fixed value
+    .byte 0x96
+
+    // Main unit code
+    .byte 0x00
+
+    // Device type (0x00 retail, 0x80 debug)
+    .byte 0x00
+
+    // Reserved
+    .byte 0x00,0x00,0x00,0x00,0x00,0x00,0x00
+
+    // Software version
+    .byte 0x00
+
+    // Complement check
+    .byte 0x51
+
+    // Reserved area
+    .space 98
+
+start_vector:
+    // Configure stacks
+    mov     r0, #0x12                      // Switch to IRQ Mode
+    msr     cpsr, r0
+    ldr     sp, =_stack_top_irq            // Set IRQ stack
+    mov     r0, #0x1f                      // Switch to System Mode
+    msr     cpsr, r0
+    ldr     sp, =_stack_top                // Set user stack
+
+    // Configure interrupt handler
+    mov     r0, #0x4000000                 // REG_BASE
+    ldr     r1, =handleInterruptARM
+    str     r1, [r0, #-4]                  // actually storing to 0x03007FFC due to mirroring
+
+    // Enable interrupts
+    mov     r1, #1
+    str     r1, [r0, #0x208]               // 0x04000208 Interrupt Master Enable
+
+    // Jump to user code (switching to Thumb mode)
+    ldr     r3, =main
+    bx      r3
+
+// Small interrupt handler that immediately jumps to a function defined in the
+// program (in Thumb) for further processing.
+handleInterruptARM:
+    ldr     r0, =handleInterrupt
+    bx      r0