materialize string literals into memory(string)

[mbenke]

string is a comptime-only type with no runtime representation. String literals and comptime concatenations are now polymorphic via a primitive Str class and are materialized into memory(string) at runtime sites, mirroring the comptime integer/Int.fromInteger design.

• StrLiteralDesugar: untyped pre-typecheck pass wrapping every string literal and every concatLit(...) in Str.fromString(...), resolved by the type checker per use site (identity at string, materialization at memory(string)). Wrapping concatLit makes it effectively result-polymorphic, so concatLit(...) (incl. nested) works at a memory(string) site without a manual Str.fromString.
• Primitives/TcEnv/NameResolution: Str class with bodyless string and memory(string) instances; memStringFromLit backend marker.
• Specialise: Str.fromString resolves by result type to identity or memStringFromLit.
• MastEval: Str.fromString identity fold; memStringFromLit kept impure.
• EmitHull: intercept memStringFromLit [StrLit] -> per-literal, content- deduped __strlit_<n> allocator (alloc + length + char words); clean guards when a comptime-only type reaches emission.
• TcStmt: generalize markIntegerComptime -> markComptimeOnly (integer+string).
• ComptimeCheck: classify comptime-only-typed params/returns as comptime, so concatLit and string functions are implicitly comptime without cascading the annotation to callers.

Tests: comptime materialization (basic, concat, nested, dedup, negative) and an end-to-end dispatch case returning "Hello, world!" verified via evmone. Documented in doc/comptime-string.md.