add noreturn and unreachable

LANGUAGE.md

@@ -25,7 +25,7 @@ roadmap and milestone history.
 
 ### scalar, aggregate, and pointer types
 
-- exact-width integers, concrete pointer-sized `isize` / `usize`, `f32`, `f64`, `bool`, `void`, and `anyopaque`; contextual `int` accepts the whole integer family, while `uint` accepts only unsigned native and target-classified C integers
+- exact-width integers, concrete pointer-sized `isize` / `usize`, `f32`, `f64`, `bool`, `void`, `noreturn`, and `anyopaque`; `noreturn` is a bottom type valid as a native function result and coerces to any expected value type; contextual `int` accepts the whole integer family, while `uint` accepts only unsigned native and target-classified C integers
 - target-dependent C scalar primitives from `c_char` through `c_longdouble`, kept semantically distinct from native scalars
 - contextual integer/float/character literals, backward type-demand inference through names and arithmetic/bitwise expressions, and typed compile-time evaluation for numeric constant expressions
 - strict numeric conversion by default, widening where valid, C scalar coercions at C boundaries, and explicit scalar keyword casts such as `i32(x)` / `c_float(x)`
@@ -98,6 +98,7 @@ fields. `_` is not a keyword member name.
 - `for` loops over ranges, arrays, slices, and pointers-to-arrays with copy captures, pointer captures `|@item|`, and optional `usize` index captures; `expand for` specializes a comptime aggregate into one checked body per element
 - `break`, `continue`, labeled `break :label`, labeled `continue :label`, and labeled plain blocks; `break :label` can cross nested scopes to exit a labeled block
 - bare block scopes, `defer`, and fallible-function `errdefer` with optional error capture; cleanup is block-scoped and LIFO
+- always-trapping `unreachable`, a `noreturn` expression that diagnoses use during comptime evaluation and terminates the current runtime path
 - bare void `return`, same-line `return value`, value blocks, value `if` with implicit single-expression branches, value loops, value `match`, and strictly value-producing `yield value` / `yield :label value`
 - `match` statements/expressions over enums, tagged unions, and scalars, including exhaustiveness checks, payload captures, pointer payload captures, multi-pattern arms, and scalar range patterns
 - a final `expand |value|:` enum arm or `expand |payload[, tag]|:` tagged-union arm generates one specialized arm for each variant not covered earlier; enum values and optional tags are comptime-known, while union payloads keep their concrete variant type