Add Nix flake for mcrctl

Vendor dependencies and expose mcrctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/modernc.org/libc/libc_windows_arm64.s

@@ -0,0 +1,20 @@
+#include "textflag.h"
+
+// func callStrtod(fn uintptr, s uintptr, p uintptr) float64
+TEXT ·callStrtod(SB), NOSPLIT, $0
+	// 1. Load arguments from Go stack
+	// Go passes args on stack for ABI0 (assembly functions).
+	MOVD	fn+0(FP), R2	// Function address (use R2 as temp)
+	MOVD	s+8(FP), R0	// Arg 1: str -> x0 (R0 in Go ASM)
+	MOVD	p+16(FP), R1	// Arg 2: endptr -> x1 (R1 in Go ASM)
+
+	// 2. Call the C function
+	// BL (Branch with Link) to the address in R2
+	CALL	R2
+
+	// 3. Handle Return Value
+	// C returns double in d0 (F0 in Go ASM).
+	// Go expects return value at ret+24(FP).
+	FMOVD	F0, ret+24(FP)
+	
+	RET