expose user‑registration in the Swift bindings

Nobody has claimed this yet.

Problem

The Swift package generated from the Matrix Rust SDK lets apps log in (password, email + password, JWT, OIDC, QR, …) but offers no way to create a new account. Developers must either:

hand‑roll a /_matrix/client/v3/register REST call and then log in, or

rely on an external browser/OIDC page.

This breaks first‑run UX and duplicates work already solved inside the core SDK.

What’s missing

Password / e‑mail registration (UIAA flow for /_matrix/client/v3/register)

OIDC self‑service registration (MSC 2965, already supported in Rust via PR matrix-rust-sdk #4169)

Neither surface in the current FFI, so they can’t be used from Swift.

Why it matters

Enables true “sign‑up & go” onboarding in native iOS/macOS apps.

Eliminates boilerplate REST code and browser detours.

Brings parity with Element Web/Android where registration is SDK‑driven.

References

Rust PR adding OIDC registration: https://github.com/matrix-org/matrix-rust-sdk/pull/4169

Matrix spec: POST /_matrix/client/v3/register, MSC 2965

Please consider exposing these registration paths through the FFI so the Swift package can offer a complete authentication story.

Start by tracing the existing Swift authentication FFI surfaces and the Rust registration support referenced in matrix-rust-sdk PR #4169. Compare the current bindings with the /_matrix/client/v3/register UIAA flow and MSC 2965, then verify that both password/email and OIDC registration are available through the Swift package.

Written by the indexing model from the issue text.