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)

examples/strings.bux

@@ -0,0 +1,34 @@
+// Strings - String manipulation with Std::String
+import Std::Io::{PrintLine, PrintInt};
+import Std::String::{String_Len, String_Eq, String_Concat};
+
+func Main() -> int {
+    let hello: String = "Hello";
+    let world: String = "World";
+    
+    PrintLine("String operations:");
+    
+    // Length
+    PrintLine("Length of 'Hello':");
+    PrintInt(String_Len(hello));
+    PrintLine("");
+    
+    // Concatenation
+    let greeting: String = String_Concat(hello, ", ");
+    let greeting2: String = String_Concat(greeting, world);
+    let full: String = String_Concat(greeting2, "!");
+    
+    PrintLine("Concatenated:");
+    PrintLine(full);
+    
+    // Equality
+    if String_Eq(hello, "Hello") {
+        PrintLine("'Hello' equals 'Hello'");
+    }
+    
+    if !String_Eq(hello, world) {
+        PrintLine("'Hello' does not equal 'World'");
+    }
+    
+    return 0;
+}