…er`) is not discarded, causing memory leaks and potentially corrupting the batch linked list.

This commit fixes the issue reported at packages/svelte/src/internal/client/reactivity/batch.js:1464

## Bug Explanation

The bug occurs in the `fork()` function's `discard` method in `batch.js`.

When using `$state.eager()` within a fork, the `eager_flush()` function creates a new Batch for the eager work:

```javascript
function eager_flush(batch) {
    flushSync(() => {
        const eager_batch = (current_batch = batch?.eager ?? new Batch());
        if (batch?.is_fork) {
            eager_batch.is_fork = true;  // Set to true, meaning #is_deferred() returns true
            batch.eager = eager_batch;   // Stored on the fork's batch
        }
        // ...
    });
}
```

This eager batch:
1. Is linked into the global batch linked list via `new Batch()`
2. Has `is_fork = true` set, which causes `#is_deferred()` to return true
3. Will never commit or unlink itself because `#is_deferred()` prevents progression

In the fork's `commit` method, this is properly handled:
```javascript
if (batch.eager) {
    const eager = batch.eager;
    eager.is_fork = false;  // Clear the flag
    // ... flush the batch ...
    eager.flush();
    batch.eager = null;  // Clear the reference
}
```

But in the `discard` method (before the fix), only `batch.discard()` was called:
```javascript
discard: () => {
    // ...
    if (!committed && batch.linked) {
        batch.discard();  // Only the main batch is discarded!
    }
}
```

This leaves the eager batch:
- Still linked in the batch list
- With `is_fork = true` (can never complete)
- Never cleaned up (memory leak)
- Potentially corrupting future batch operations

## Fix Explanation

The fix adds `batch.eager?.discard()` before `batch.discard()` in the fork's discard method:

```javascript
if (!committed && batch.linked) {
    batch.eager?.discard();  // Also discard the eager batch if it exists
    batch.discard();
}
```

This ensures that when a fork is discarded, any associated eager batch is also properly discarded and unlinked from the batch list, preventing memory leaks and list corruption.

Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>
Co-authored-by: Rich-Harris <hello@rich-harris.dev>