fix(qbe): any-child terminator check in QBE_IsTerminator for hBlock — QBE SSA now fully valid

- QBE_IsTerminator(hBlock) checks ALL children, not just last
- Removed legacy c_backend.bux and nim_backend.bux from _selfhost/src/
- QBE self-hosting QBE phase: PASSES (SSA valid, assembled to .s)
- Remaining: linker errors from missing String_Eq/StringBuilder in runtime

vendor/qbe/amd64/all.h

@@ -0,0 +1,70 @@
+#include "../all.h"
+
+typedef struct Amd64Op Amd64Op;
+
+enum Amd64Reg {
+	RAX = RXX+1, /* caller-save */
+	RCX,
+	RDX,
+	RSI,
+	RDI,
+	R8,
+	R9,
+	R10,
+	R11,
+
+	RBX, /* callee-save */
+	R12,
+	R13,
+	R14,
+	R15,
+
+	RBP, /* globally live */
+	RSP,
+
+	XMM0, /* sse */
+	XMM1,
+	XMM2,
+	XMM3,
+	XMM4,
+	XMM5,
+	XMM6,
+	XMM7,
+	XMM8,
+	XMM9,
+	XMM10,
+	XMM11,
+	XMM12,
+	XMM13,
+	XMM14,
+	XMM15,
+
+	NFPR = XMM14 - XMM0 + 1, /* reserve XMM15 */
+	NGPR = RSP - RAX + 1,
+	NGPS = R11 - RAX + 1,
+	NFPS = NFPR,
+	NCLR = R15 - RBX + 1,
+};
+MAKESURE(reg_not_tmp, XMM15 < (int)Tmp0);
+
+struct Amd64Op {
+	char nmem;
+	char zflag;
+	char lflag;
+};
+
+/* targ.c */
+extern Amd64Op amd64_op[];
+
+/* sysv.c (abi) */
+extern int amd64_sysv_rsave[];
+extern int amd64_sysv_rclob[];
+bits amd64_sysv_retregs(Ref, int[2]);
+bits amd64_sysv_argregs(Ref, int[2]);
+void amd64_sysv_abi(Fn *);
+
+/* isel.c */
+void amd64_isel(Fn *);
+
+/* emit.c */
+void amd64_emitfn(Fn *, FILE *);