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

Makefile

@@ -5,15 +5,19 @@ BUILD_DIR := build
 
 EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics generics_struct generic_infer generic_infer2 extend_generic pattern_matching strings strings2 map result_option try_operator ownership ctfe async concurrency os_time process json iter
 
-.PHONY: all build dev test clean test-examples
+.PHONY: all build dev debug test clean clean-all test-examples selfhost
 
 all: build
 
 build:
-	$(NIM) c -o:$(OUT) -d:release $(SRC)
+	$(NIM) c -o:$(OUT) -d:release --opt:size $(SRC)
+	strip $(OUT)
 
 dev:
-	$(NIM) c -o:$(OUT) $(SRC)
+	$(NIM) c -o:buxc_debug -d:debug --stackTrace:on --lineTrace:on $(SRC)
+
+debug: dev
+	@echo "Debug binary: buxc_debug"
 
 test: build test-examples
 	@echo "Running lexer tests..."
@@ -50,17 +54,22 @@ test-examples: build
 	@echo "All examples passed!"
 
 clean:
-	rm -f $(OUT)
+	rm -f $(OUT) buxc_debug
 	rm -rf $(BUILD_DIR)
 	rm -rf nimcache
 	rm -rf examples_pkg
 	rm -rf _test_tmp_pkg
+	rm -rf _test_cast _test_cast2 _test_cast3 _test_channel
+
+clean-all: clean
+	rm -rf _selfhost/src _selfhost/build
 
 selfhost: build
-	@echo "=== Phase 7.9: Building self-hosted compiler ==="
-	@rm -rf _selfhost/src
+	@echo "=== Building self-hosted compiler ==="
+	@rm -rf _selfhost/src _selfhost/build
 	@mkdir -p _selfhost/src
 	@cp compiler/selfhost/*.bux _selfhost/src/
 	@mv _selfhost/src/main.bux _selfhost/src/Main.bux 2>/dev/null || true
 	@cd _selfhost && ../$(OUT) build
+	@strip _selfhost/build/buxc2
 	@echo "=== Self-hosted compiler built successfully ==="