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/Result.bux

@@ -0,0 +1,44 @@
+module Std::Result {
+import Std::Io::{PrintLine};
+
+enum Result {
+    Ok(int),
+    Err(String),
+}
+
+func Result_NewOk(value: int) -> Result {
+    let r: Result = Result { tag: Result_Ok };
+    r.data.Ok_0 = value;
+    return r;
+}
+
+func Result_NewErr(msg: String) -> Result {
+    let r: Result = Result { tag: Result_Err };
+    r.data.Err_0 = msg;
+    return r;
+}
+
+func Result_IsOk(r: Result) -> bool {
+    return r.tag == Result_Ok;
+}
+
+func Result_IsErr(r: Result) -> bool {
+    return r.tag == Result_Err;
+}
+
+func Result_Unwrap(r: Result) -> int {
+    if r.tag != Result_Ok {
+        PrintLine("panic: unwrap on Err");
+        return 0;
+    }
+    return r.data.Ok_0;
+}
+
+func Result_UnwrapOr(r: Result, fallback: int) -> int {
+    if r.tag == Result_Ok {
+        return r.data.Ok_0;
+    }
+    return fallback;
+}
+
+}