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)

stdlib/runtime.c

@@ -127,3 +127,38 @@ void bux_bounds_check(size_t index, size_t len) {
         abort();
     }
 }
+
+/* String wrappers with Bux-compatible signatures */
+unsigned int bux_strlen(const char* s) {
+    return (unsigned int)strlen(s);
+}
+
+int bux_strcmp(const char* a, const char* b) {
+    return strcmp(a, b);
+}
+
+int bux_strncmp(const char* a, const char* b, unsigned int n) {
+    return strncmp(a, b, (size_t)n);
+}
+
+char* bux_strcpy(char* dest, const char* src) {
+    return strcpy(dest, src);
+}
+
+char* bux_strcat(char* dest, const char* src) {
+    return strcat(dest, src);
+}
+
+char* bux_strncpy(char* dest, const char* src, unsigned int n) {
+    return strncpy(dest, src, (size_t)n);
+}
+
+/* Hash function (djb2) for string keys */
+unsigned int bux_hash_string(const char* s) {
+    unsigned int hash = 5381;
+    int c;
+    while ((c = *s++)) {
+        hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
+    }
+    return hash;
+}