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/README

@@ -0,0 +1,18 @@
+QBE - Backend Compiler              http://c9x.me/compile/
+
+doc/    Documentation.
+minic/  An example C frontend for QBE.
+tools/  Miscellaneous tools (testing).
+test/   Tests.
+amd64/
+arm64/
+rv64/   Architecture-specific code.
+
+The LICENSE file applies to all files distributed.
+
+- Compilation and Installation
+
+Invoke make in this directory to create the executable
+file qbe.  Install using 'make install', the standard
+DESTDIR and PREFIX environment variables are supported.
+Alternatively, you may simply copy the qbe binary manually.