refactor: remove QBE backend, improve bootstrap compiler and stdlib

Bootstrap compiler improvements:
- Extract findStdlibDir() to remove hardcoded path and deduplicate
- Extract prepareProject()/mergeProject() to share code between cmdCheck/cmdBuild
- Fix C backend to emit warning on unknown types instead of silent 'int'
- Clean up all unused imports across 7 modules
- Makefile: add strip, debug target, clean-all, selfhost strip

QBE removal:
- Remove vendor/qbe/ (74K lines)
- Remove compiler/selfhost/qbe_backend.bux, nim_backend.bux

Stdlib improvements:
- Add Result.bux + Option.bux modules
- Fix Json.bux memory leak (removed double String_Copy)
- Add missing imports in Crypto.bux (Alloc/Free) and Test.bux (PrintLine/PrintInt)
- Clean stale buxc_debug binary

library/std/Json.bux

@@ -274,8 +274,7 @@ func JsonParser_ParseString(p: *JsonParser) -> String {
         return "";
     }
     JsonParser_Advance(p);
-    let raw: String = StringBuilder_Build(&sb);
-    let result: String = String_Copy(raw);
+    let result: String = StringBuilder_Build(&sb);
     StringBuilder_Free(&sb);
     return result;
 }