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)

src/c_backend.nim

@@ -235,6 +235,10 @@ proc emitExpr(be: var CBackend, node: HirNode): string =
   of hIs:
     return "true"  # TODO: proper type checking
 
+  of hSizeOf:
+    let typ = typeToC(node.sizeOfType)
+    return &"sizeof({typ})"
+
   of hIf:
     # Ternary expression
     let cond = be.emitExpr(node.ifCond)