feat: execute mutable type-valued compile-time procedures

README.md

@@ -96,7 +96,9 @@ main proc() i16 {
 }
 ```
 
-the specialization checkpoint supports type arguments and constant integer, boolean, string, callback and null value arguments; declared value-parameter types survive specialization, including narrow checked arithmetic and optional captures/unwrap. integer arguments are range-checked even when unused, and erased argument expressions still receive ordinary checking. immutable constant expressions may use unary signs, `!`, `+`, `-`, `*`, comparisons and logical operators. type-producing procedures accept ordinary parameters as well as `$` parameters: every argument must be supplied explicitly and evaluate to a constant, and every input participates in specialization caching. for example, `Fixed proc(T type, count usize) type { return struct { values [count]T } }` accepts `Fixed(i16, 2)` and reuses its nominal type for equivalent inputs. use `$` when a parameter must be visible in subsequent signature annotations. type-producing bodies support immutable locals, conditional returns, nested type-producing calls and generated record/array storage, including pointer-recursive records; mutable type-valued locals and loops in these bodies remain unsupported. runaway specialization is diagnosed at 128 nested evaluation/specialization calls or 4096 instances.
+the specialization checkpoint supports type arguments and constant integer, boolean, string, callback and null value arguments; declared value-parameter types survive specialization, including narrow checked arithmetic and optional captures/unwrap. integer arguments are range-checked even when unused, and erased argument expressions still receive ordinary checking. immutable constant expressions may use unary signs, `!`, `+`, `-`, `*`, comparisons and logical operators. type-producing procedures accept ordinary parameters as well as `$` parameters: every argument must be supplied explicitly and evaluate to a constant, and every input participates in specialization caching. for example, `Fixed proc(T type, count usize) type { return struct { values [count]T } }` accepts `Fixed(i16, 2)` and reuses its nominal type for equivalent inputs. use `$` when a parameter must be visible in subsequent signature annotations. runaway specialization is diagnosed at 128 nested evaluation/specialization calls or 4096 instances.
+
+type-producing bodies support mutable type and value locals, direct binding assignment, scoped blocks, conditional/early returns, integer range and while loops, break/continue and nested/recursive type-producing calls. for example, start with `Result := T`, repeat `Result = [2]Result` in a loop, then `return Result` to generate nested array storage. snapshots retain each local's current value without changing the source AST; scalar operations reuse checking, typed IR and interpreter execution, preserving exact types and overflow diagnostics. type-control execution is limited to 4096 statement/loop-guard steps shared with nested producers; individual scalar evaluations retain the interpreter limits below. only executed bodies are elaborated, while every body is still parsed and name-resolved. field/index assignment, optional captures, cleanup and type-valued aggregate storage in type-producing bodies remain outside this checkpoint.
 
 ordinary procedures may compute integer, boolean, float and string compile-time arguments and return nested records/tuples and fixed arrays from constant inputs, such as `Fixed(i16, length(2))`. calls reuse the ordinary checker, typed IR and interpreter, preserving local type inference, checked arithmetic, mutable locals, range/while loops, break/continue and recursive or specialized helper calls. aggregate arguments receive their declared context, including dependent `[N]T` parameters; cached specializations compare normalized values, not literal spelling or evaluation identity. returned aggregates become owned source literals before temporary interpreter storage is released, preserving by-value copies, exact integer types, string bytes, enum fields, optional fields and IEEE float bits.