Response helpers are inconsistent about mutate versus return

## What happened?

The response helpers are inconsistent about whether they mutate or return, and the signatures do not match the behaviour. Every helper that declares a return value returns **the same object it was given** — none returns a new one — so the type reads like a transformation when it is a mutation.

Measured against the current `main`:

| helper | declared | actual |
|---|---|---|
| `fatal` | `RunFunctionResponse` | mutates, returns **the same object** |
| `normal` | *(inferred void)* | mutates, returns **void** |
| `warning` | *(inferred void)* | mutates, returns **void** |
| `setDesiredCompositeStatus` | `RunFunctionResponse` | mutates, returns **the same object** |
| `setDesiredComposedResources` | `RunFunctionResponse` | mutates, returns **the same object** |
| `setContextKey` | `RunFunctionResponse` | mutates, returns **the same object** |

Note that `fatal` and its siblings `normal` / `warning` disagree with each other, so this is not even a clean split along "results helpers" versus "setters".

### Why it matters more now

Under the class-based `FunctionHandler`, `rsp` was a local, so `rsp = setDesiredCompositeStatus({ rsp, status })` hid the ambiguity — reassigning a local is harmless whether or not the value is new.

With `ComposeFunction` (#32) `rsp` is a parameter. Respecting the declared contract means introducing a variable:

```typescript
const withStatus = setDesiredCompositeStatus({ rsp, status });
return withStatus;
```

…and `withStatus === rsp`. That variable exists only because the signature is ambiguous. Writing the honest version instead — call it, then `return rsp` — looks like a bug to anyone who trusts the type.

## How can we reproduce it?

```typescript
import { to, fatal, normal, setDesiredCompositeStatus } from '@crossplane-org/function-sdk-typescript';

const rsp = to(req);
console.log(fatal(rsp, 'm') === rsp); // true
console.log(normal(rsp, 'm')); // undefined
console.log(setDesiredCompositeStatus({ rsp, status: {} }) === rsp); // true
```

## Suggested fix

Pick one and apply it across the board:

1. **Return void everywhere.** Honest about what these do, and the smallest change in behaviour. Breaking for anyone currently chaining or reassigning.
2. **Genuinely return new objects.** Matches the current types, and composes nicely, but is a real behavioural change and more allocation per call.

Either is breaking, so it wants a minor version and a note in the release.

## Related: `to()` aliases the request's desired state

Separately, `to()` assigns `req.desired` into the response rather than copying it:

```typescript
const rsp = to(req);
rsp.desired === req.desired; // true, when the request has desired state
rsp.desired.resources['x'] = something; // also mutates req.desired.resources
```

Pre-existing, and mostly harmless because functions read observed state and write desired state. But `ComposeFunction` makes `rsp.desired` the primary write surface, so it is now much easier to hit. #32 documents the behaviour on `ComposeFunction` and pins it with a test rather than changing it, since copying is a behavioural change that belongs with the decision above.

Start by locating the TypeScript entry points for fatal, normal, warning, setDesiredCompositeStatus, setDesiredComposedResources, setContextKey, and to(), then review ComposeFunction (#32) and its tests. Resolve whether the helpers consistently return void or new objects, apply the decision across their signatures and behavior, and add the required tests and release note for the breaking change.

索引モデルが issue の本文から書いたものです。