perry-codegen: String.startsWith 2-arg form (with position) unsupported #315

What happened

Perry's codegen rejects the standard ES String.prototype.startsWith(searchString, position) 2-arg form with:

perry-codegen: String.startsWith expects 1 arg, got 2

Surfaced compiling effect/src/String.ts (func 19) during the #309 compat sweep.

What you expected

MDN spec for String.prototype.startsWith: the 2-arg form is standard and widely used. Perry should accept either arity.

Minimal reproduction

const s = "abcdef";
console.log(s.startsWith("cd", 2));   // expected: true
console.log(s.startsWith("ab", 0));   // expected: true

perry compile repro.ts -o /tmp/out
# fails with: perry-codegen: String.startsWith expects 1 arg, got 2

Likely fix site

crates/perry-codegen/src/lower_string_method.rs — search for startsWith arm. Should mirror what slice / substring already do for optional second arg (default to 0 when absent, lowering to a js_string_starts_with_at(s, prefix, pos) runtime call).

String.prototype.endsWith(s, endPosition) likely has the same gap and should be fixed in the same change — worth a grep for endsWith expects in the codebase.

Environment

• Perry version: 0.5.405
• Host OS: macOS 26.4 (arm64)
• Target: native
• Effect version: 3.21.2 (where this surfaced)

Discovered as part of the #309 long tail.