feat(typecheck): String relational ops (<, >, <=, >=) (closes #458)

[hyperpolymath]

Summary

Closes #458 — `String < String` (and `>` / `<=` / `>=`) now type-check, lowering to JS's native lexicographic string comparison. Pre-fix: `TypeMismatch (String, Int)`.

Implementation

Single addition to the existing comparison dispatch in `Typecheck.synth_expr` for `ExprBinary`:

```ocaml
match repr lhs_ty with
| TCon "Float" -> ...
| TCon "String" ->
let* () = check ctx rhs ty_string in
Ok ty_bool
| _ -> ... (* legacy Int monomorphism *)
```

Pattern mirrors the existing Float dispatch a few lines up. No codegen changes needed — JavaScript's `<` / `>` / `<=` / `>=` on strings is lex compare natively, and the JS-family backends already emit those operators verbatim.

Test plan

New regression fixture `tests/codegen-deno/string_lex_cmp.affine` + harness with 22 assertions:

• All four ops via functional form (`lt(a, b)`, etc.) — covers each operator's positive/negative direction

• All four ops via literal form (`first_lt()`, etc.)

• Equal-string corner cases — `x <= x` true, `x >= x` true, `x < x` false

• Empty strings — `"" < "a"`, `"" <= ""`

• Prefix relations — `"abc" < "abcd"`

• Local `./tools/run_codegen_deno_tests.sh`: 14/14 harnesses green

• Local `dune test`: 352/352 green

• Smoke compile: `return a < b;` emits as `return (a < b);` (JS native)

Out of scope

• Non-ASCII string comparison in the fixture: this branch forked from `main` before fix(codegen-js): emit \uXXXX/\u{X} for non-ASCII string literals (closes #460) #463 (the companion Unicode-escape codegen fix for codegen --deno-esm: non-ASCII string literals lower to octal escapes, blocked by strict-mode ESM #460) lands, so non-ASCII source literals would still emit OCaml-style `\NNN` octal escapes that strict-mode ESM rejects. The relational typecheck change is orthogonal to literal encoding — non-ASCII lex compare works naturally once both PRs merge. A non-ASCII assertion can be added in a follow-up commit after fix(codegen-js): emit \uXXXX/\u{X} for non-ASCII string literals (closes #460) #463 merges, or auto-rebased here if they land in either order.
• Other backends (rescript, wasm, lua, c, rust): out of scope; lexer/typecheck: String < String not built-in for lex comparison #458 specifically called out the JS-family ergonomic gap. If `String <` lowering for other backends becomes load-bearing, file separately.

Refs

• Closes lexer/typecheck: String < String not built-in for lex comparison #458
• Refs feat(scripts): AffineScript port of check-ts-allowlist (TS->AS campaign STEP 2 seed) standards#284 (the seam-analyst PR with the `str_lt` workaround)
• Companion: fix(codegen-js): emit \uXXXX/\u{X} for non-ASCII string literals (closes #460) #463 (codegen --deno-esm: non-ASCII string literals lower to octal escapes, blocked by strict-mode ESM #460 Unicode-escape codegen, lands together to unblock non-ASCII relational comparisons)

🤖 Generated with Claude Code

[github-actions]

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