getAssignment: external experiment assignment (don't randomize)

Part of #66

## Problem

Experiments self-bucket client-side (`ued.evaluateDecisionPolicy`); the only override is a `?experiment=id/variant` query param. There is no clean way to say *"the arm comes from my engine — don't randomize"* while still letting the plugin render the variant and report. This was the biggest gap: a BYO experiment currently has to be modeled as an audience/campaign instead.

## Proposal

A pluggable assignment provider the engine owns:

```js
loadEager(document, {
getAssignment: async (experimentId, context) => variant, // e.g. 'challenger-1' | 'control'
});
```

When provided, the plugin **skips client-side randomization** for that experiment and renders + reports the returned variant. Sticky assignment and exposure stay the engine's responsibility.

## Acceptance criteria

- [ ] When `getAssignment` returns a variant, the plugin serves it and does not re-bucket.
- [ ] When absent (default), current self-bucketing is unchanged.
- [ ] `?experiment=` override still wins for QA.
- [ ] Works across the experiment fidelity levels the plugin already supports.

Start at the loadEager entry point and trace how ued.evaluateDecisionPolicy, the experiment query-parameter override, and existing fidelity levels choose a variant. Verify the assignment-provider path skips re-bucketing, preserves the default behavior and QA override, and serves and reports the returned variant across supported fidelity levels.

Written by the indexing model from the issue text.