[labs/virtualizer] `averageMarginSize` in metrics cache is off

### Which package(s) are affected?

Virtualizer (@lit-labs/virtualizer)

### Description

Given a list of 10000 items and each item having a height of 32px and a bottom margin of 2px, the `averageMarginSize` is not 2px, but ~1.95px. This is due to the first item:

```
const a = this._metricsCache.get(k)?.[leadingMargin(direction)] || 0;
const b = this._metricsCache.get(k - 1)?.[trailingMargin(direction)] || 0;
this._marginSizeCache.set(k, collapseMargins(a, b));
```

For the first item (`k === 0`), `b` falls through and becomes `|| 0`.

If i scroll to item with index 3000 in the list, this causes a delta of `102034 - 101893 = 141px`.

Perhaps the margin size of the first item should not be cached so as not to "dirty" the cache?

Also, usually the first item in a list has no top margin.

### Reproduction

-

### Workaround

-

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

Latest

### Browser/OS/Node environment

-

Start in the @lit-labs/virtualizer metrics-cache code around _metricsCache and the margin-size calculation shown in the issue. Reproduce the 10,000-item list with 32px heights and 2px bottom margins, then check the first-item handling and scrolling to item 3000. Done means averageMarginSize reflects the expected margin and the reported scroll delta is resolved.

Written by the indexing model from the issue text.