renderDecision: pluggable decision renderer

Part of #66

## Problem

Applying a decision is hard-coded to *fetch URL → `innerHTML`* with a fixed `main > div` fallback. Real engines return JSON, content references, or external-CMS ids — none of which fit that single shape, so every integrator re-implements the application seam.

## Proposal

```js
loadEager(document, {
renderDecision: async (el, decision) => { /* apply however the decision is shaped */ },
});
```

Default (unset) keeps the current fetch-and-replace behavior. When provided, the plugin hands the target element + the normalized decision to the integrator (e.g. json2html, external fetch, DOM patch).

## Acceptance criteria

- [ ] `renderDecision` is used to apply the decision when provided.
- [ ] Default behavior (fetch URL → replace) unchanged when unset.
- [ ] Receives the resolved element and the normalized decision object.

Start at the loadEager implementation and trace the current fetch-and-replace path, including how the target element and normalized decision are resolved. Verify that a provided renderDecision receives both values and controls application, while the unset case preserves the existing behavior; run the existing tests and add coverage if the repository has tests for this path.

Written by the indexing model from the issue text.