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/Std/String.bux

@@ -0,0 +1,45 @@
+module Std::String {
+
+extern func bux_strlen(s: String) -> uint;
+extern func bux_strcmp(a: String, b: String) -> int;
+extern func bux_strncmp(a: String, b: String, n: uint) -> int;
+extern func bux_strcpy(dest: *char8, src: String) -> *char8;
+extern func bux_strcat(dest: *char8, src: String) -> *char8;
+extern func bux_strncpy(dest: *char8, src: String, n: uint) -> *char8;
+
+func String_Len(s: String) -> uint {
+    return bux_strlen(s);
+}
+
+func String_Eq(a: String, b: String) -> bool {
+    return bux_strcmp(a, b) == 0;
+}
+
+func String_Concat(a: String, b: String) -> String {
+    let len_a: uint = bux_strlen(a);
+    let len_b: uint = bux_strlen(b);
+    let total: uint = len_a + len_b + 1;
+    let buf: *char8 = bux_alloc(total) as *char8;
+    bux_strcpy(buf, a);
+    bux_strcat(buf, b);
+    return buf;
+}
+
+func String_Copy(s: String) -> String {
+    let len: uint = bux_strlen(s);
+    let buf: *char8 = bux_alloc(len + 1) as *char8;
+    bux_strcpy(buf, s);
+    return buf;
+}
+
+func String_StartsWith(s: String, prefix: String) -> bool {
+    let s_len: uint = bux_strlen(s);
+    let p_len: uint = bux_strlen(prefix);
+    if p_len > s_len {
+        return false;
+    }
+    let r: int = bux_strncmp(s, prefix, p_len);
+    return r == 0;
+}
+
+}