lexer/typecheck: String < String not built-in for lex comparison

[hyperpolymath]

Surfaced by

hyperpolymath/standards#284 — AffineScript port of `check-ts-allowlist.ts` (TS->AS migration STEP 2 tail-batch-1, hyperpolymath/standards#239 umbrella).

Behaviour

```affine
let a: String = "abc"
let b: String = "abd"
if a < b { ... } // ERROR: TypeMismatch (String, Int)
```

`<` / `>` / `<=` / `>=` work for `Int` and `Float` (unified per the no-OCaml-float-ops convention) but do not have a `String` overload for lexicographic comparison.

Why this hurts

Several scripts in the estate sort string lists or compare path prefixes byte-by-byte. The standards#284 port had to inline a manual `str_lt` helper using `char_to_int(string_get(...))` for every `<` use site. Every future TS->AS port that touches sort/order will hit the same wall.

Proposed fix

Add String overloads for `< > <= >=` that lower to byte-wise lex compare. Pattern follows the existing `String ==` / `!=` overloads. Should resolve in `Typecheck.binop_resolve` (or wherever the existing integer-comparison handler lives).

Alternative: ship a `String.lt` / `String.compare` stdlib function in `stdlib/String.affine`. Worse ergonomics (`String.lt a b` vs `a < b`) but smaller compiler change.

Acceptance

• `let _ = "a" < "b"` type-checks and emits a working comparison
• Tests cover empty-string, equal-prefix, and non-ASCII edge cases
• One of: typecheck overload OR `String.compare` stdlib (owner choice)

Refs

• feat(scripts): AffineScript port of check-ts-allowlist (TS->AS campaign STEP 2 seed) standards#284 (work-around in `scripts/check-ts-allowlist.affine` `str_lt`)
• [campaign] TypeScript → AffineScript estate migration (UMBRELLA) standards#239 (TS->AS campaign umbrella)