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_386.s

@@ -0,0 +1,29 @@
+#include "textflag.h"
+
+// func callStrtod(fn uintptr, s uintptr, p uintptr) float64
+TEXT ·callStrtod(SB), NOSPLIT, $0
+	// 1. Initialize FPU
+	// This ensures the x87 stack is empty (Tag Word = FFFF).
+	// Without this, garbage on the stack causes the result push to overflow -> NaN.
+	FINIT
+
+	// 2. Load arguments from Go stack
+	MOVL	fn+0(FP), AX	// Function pointer
+	MOVL	s+4(FP), CX	// String pointer
+	MOVL	p+8(FP), DX	// Endptr pointer
+
+	// 3. Setup C stack for __cdecl
+	SUBL	$8, SP
+	MOVL	DX, 4(SP)	// Push endptr
+	MOVL	CX, 0(SP)	// Push str
+
+	// 4. Call the C function
+	CALL	AX
+
+	// 5. Clean up stack
+	ADDL	$8, SP
+
+	// 6. Store FPU result (ST0) into Go return slot
+	FMOVD	F0, ret+12(FP)
+	
+	RET