✨ add allSettled() operation congruent with Promise.allSettled()

[rauhryan]

Motivation

Addresses #898 — Effection has all() and race() matching Promise.all() and Promise.race(), but lacks an equivalent of Promise.allSettled().

Approach

Adds a new allSettled() operation that runs all given operations concurrently and waits for every one to settle. Unlike all(), it never short-circuits on error — each result is represented as Effection's Result<T> shape ({ ok: true, value } or { ok: false, error }).

Key decisions:

• Uses Effection's Result<T> rather than PromiseSettledResult<T> so settled outcomes compose consistently with the rest of the library
• Implements allSettled() by composing existing primitives: all() over box()-wrapped operations
• Sibling operations are not halted when one fails; all members run to completion and their outcomes are preserved
• Documents Result, Ok, and Err as public API since allSettled() now exposes Result<T> directly

Files changed

• lib/all-settled.ts — allSettled() implementation and AllSettled mapped type using Result<T>
• lib/result.ts — public API docs for Result, Ok, and Err
• test/all-settled.test.ts — coverage for success, failure, teardown, and type inference using Result<T>
• docs/async-rosetta-stone.mdx and docs/spawn.mdx — updated prose for the Result<T> shape and linked result helpers
• AGENTS.md — updated guidance for allSettled() semantics

Test coverage

• Empty list resolves to []
• All fulfilled → array of { ok: true, value }
• All rejected → array of { ok: false, error } without throwing
• Mixed outcomes preserve input order
• Does not reject when one operation rejects asynchronously
• Does not halt sibling operations when one fails
• Runs teardown for all operations after completing
• Type inference matches the Result<T>-based tuple shape