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/Option.bux

@@ -0,0 +1,42 @@
+module Std::Option {
+import Std::Io::{PrintLine};
+
+enum Option {
+    Some(int),
+    None,
+}
+
+func Option_NewSome(value: int) -> Option {
+    let o: Option = Option { tag: Option_Some };
+    o.data.Some_0 = value;
+    return o;
+}
+
+func Option_NewNone() -> Option {
+    return Option { tag: Option_None };
+}
+
+func Option_IsSome(o: Option) -> bool {
+    return o.tag == Option_Some;
+}
+
+func Option_IsNone(o: Option) -> bool {
+    return o.tag == Option_None;
+}
+
+func Option_Unwrap(o: Option) -> int {
+    if o.tag != Option_Some {
+        PrintLine("panic: unwrap on None");
+        return 0;
+    }
+    return o.data.Some_0;
+}
+
+func Option_UnwrapOr(o: Option, fallback: int) -> int {
+    if o.tag == Option_Some {
+        return o.data.Some_0;
+    }
+    return fallback;
+}
+
+}