✨ feat: implement allSettled()

[rauhryan]

Motivation

Addresses thefrontside#898 by adding an Effection analogue to Promise.allSettled().

This PR is stacked on top of upstream PR thefrontside#1149, which makes Result<T> part of the documented public API.

Approach

• add allSettled() as a new combinator built from all() over box()-wrapped operations
• return Effection Result<T> values instead of promise-style fulfilled / rejected records
• export allSettled() from the public module surface
• add tests covering success, failure, ordering, teardown, and type inference
• update docs and AGENTS guidance to describe allSettled() semantics