Use Result for join_all item outcomes instead of Settled

## Problem

`DurableContext::join_all` currently represents each input future's outcome with the custom enum:

```rust
pub enum Settled {
Fulfilled(O),
Rejected(OperationError),
}
```

`Fulfilled` / `Rejected` is JavaScript `Promise.allSettled()` terminology. In Rust, the standard representation of the same success-or-failure state is `Result`. The custom enum adds another type and matching vocabulary for callers to learn without adding a distinct state or invariant.

The current return shape is effectively:

```rust
Result>, OperationError>
```

The outer `Result` represents failure of the durable combinator itself, while each inner value represents an individual operation outcome. That distinction can be retained with the standard type:

```rust
Result>, OperationError>
```

## Proposed change

Replace `Settled` in the public `join_all` result with `Result`, and update the implementation, documentation, examples, and conformance handlers accordingly.

If the nested `Result` needs additional clarity in signatures or documentation, introduce a descriptive type alias rather than a new two-variant enum:

```rust
pub type Settled = Result;
```

The alias could also provide a temporary migration path before removing the bespoke enum.

## Why this is more idiomatic Rust

- Callers use the standard `Ok` / `Err` patterns and existing `Result` combinators.
- The API avoids JavaScript Promise terminology in a Rust SDK.
- Generic code that already works with `Result` can consume item outcomes directly.
- The type communicates that there are exactly two states and that rejection carries an error.

## Compatibility

This is a breaking public-API change, so it is best considered before the preview API stabilizes. Serialized checkpoint behavior need not change; this should be a Rust-facing representation change only.

## Acceptance criteria

- [ ] `join_all` returns per-item `Result` outcomes.
- [ ] The outer combinator-level `Result` remains distinct and documented.
- [ ] Examples and conformance handlers use `Ok` / `Err`.
- [ ] `Settled::Fulfilled` and `Settled::Rejected` are removed or covered by a documented migration path.
- [ ] Checkpoint and replay wire formats remain unchanged.

Start at the public `DurableContext::join_all` API and trace its implementation, documentation, examples, and conformance handlers that use `Settled`. Check the checkpoint and replay serialization paths before changing the Rust-facing representation; done means per-item outcomes use `Result`, examples and handlers use `Ok`/`Err`, and wire formats remain unchanged.

Written by the indexing model from the issue text.