feat: Std::String, Std::Map, Result/Option, ? operator, sizeof fix, docs

Add standard library modules:
- Std::String: strlen, strcmp, concat, copy, starts_with wrappers
- Std::Map: linear-probing hash map with String keys, int values

Add error handling:
- Result and Option algebraic enum examples
- ? try operator for automatic error propagation in HIR lowering

Compiler bugfixes:
- Fix sizeof() for user-defined structs (new hSizeOf HIR node)
- Fix HIR type lowering for char8, bool8, int8, uint8, etc.
- Fix let statement lowering for pointer types (*char8 → int* bug)
- Fix PrintInt ABI mismatch (int64_t → int in io.c)
- Fix Makefile test bug (_test_tmp_pkg already exists)

Documentation:
- Rewrite README.md with features, examples, project structure
- Add docs/LanguageRef.md — complete language reference
- Add docs/Stdlib.md — standard library documentation
- Add docs/BuildAndTest.md — build and test guide

New examples: strings, map, result_option, try_operator (13 total)

Makefile

@@ -3,7 +3,7 @@ SRC := src/main.nim
 OUT := buxc
 BUILD_DIR := build
 
-EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics pattern_matching
+EXAMPLES := hello fibonacci factorial structs enums methods algebraic_enums generics pattern_matching strings map result_option try_operator
 
 .PHONY: all build dev test clean test-examples
 
@@ -25,6 +25,7 @@ test: build test-examples
 	@echo "Running HIR tests..."
 	$(NIM) c -r tests/hir_test.nim
 	@echo "Running integration tests..."
+	rm -rf _test_tmp_pkg
 	./$(OUT) new _test_tmp_pkg
 	./$(OUT) --version