io interface (first pass)

LANGUAGE.md

@@ -31,7 +31,7 @@ roadmap and milestone history.
 - UTF-8 string literals as immutable pointers to static zero-terminated byte arrays, plus raw backtick multiline strings
 - narrow immutable zero-terminated byte pointer/slice conversion to `*c_char` / `?*c_char` without general `u8`/`c_char` interchange
 - optionals with `none`, `orelse`, postfix `?`, conditional unwraps, guarded unwraps, and left-to-right short-circuiting multi-unwraps
-- nominal distinct types with exact backing construction, native enums with optional explicit integer backing, contextual enum literals, and imported C enums as target-backed integer aliases
+- nominal distinct types with exact backing construction, native enums with optional explicit integer backing and explicit backing-to-scalar casts, contextual enum literals, and imported C enums as target-backed integer aliases
 - source-order native structs, opaque nominal records with `Name :: opaque`, complete `c_struct { ... }`, keyed record literals, native untagged unions, and native tagged unions `union(Enum)` / `union(enum)`
 - void-payload tagged-union variants, anonymous struct payloads, contextual `.variant`, `.variant{payload}`, and `.variant{field = value}` construction
 - native sum composition with `A | B` for unbacked enums and tagged unions, using program-global `u16` variant ids
@@ -123,6 +123,7 @@ The six spellings are reserved only as direct unqualified calls. A qualified cal
 
 - `std/mem` generic slice equality, allocator contract with raw byte operations, typed `empty` / `alloc` / `realloc` / `free`, overflow checks, zero-sized-type support, and failure-preserving reallocation
 - `std/arraylist` generic `ArrayList(T)` with direct `items` slice access, explicit capacity, allocator ownership, fallible reserve/append, clear, and deinit
+- `std/io` explicit `Io` capabilities, `Reader`/`Writer` stream values, one-shot `read`/`write`, and allocation-free `write_all`
 
 ### compiler behavior
 
@@ -130,6 +131,7 @@ The six spellings are reserved only as direct unqualified calls. A qualified cal
 - lazy semantic checking of demanded function specializations
 - static, eager runtime, mutable runtime, and deferred problematic globals with cycle diagnostics
 - demand-driven LLVM declarations for referenced foreign functions
+- root `main` may be parameterless or accept the canonical `@std/io Io`; the injected form is called through a synthesized no-argument C entry point
 - replaceable dynamically loaded libclang C-import backend
 - C-header import caching by canonical path, target, include paths, and defines