# Fix DataView clones getting the wrong byteLength

## Problem

Cloning a `DataView` that covers only part of its underlying `ArrayBuffer`
produces a clone with the wrong `byteLength`: the copy spans from the original
`byteOffset` to the end of the buffer instead of keeping the original length.

```js
const clone = require('rfdc')()

const buffer = new ArrayBuffer(16)
const view = new DataView(buffer, 4, 8) // offset 4, length 8

const copy = clone({ a: view }).a
copy.byteLength // 12  (expected 8)
```

The value at the start of the view is copied correctly and the clone is
independent of the source, but the clone can now read 4 bytes past the end of
the original view.

## Root cause

`index.js`, in `copyBuffer`:

```js
return new cur.constructor(cur.buffer.slice(), cur.byteOffset, cur.length)
```

This path handles every `ArrayBuffer.isView` value, i.e. both TypedArrays and
`DataView`. It is correct for TypedArrays, whose third constructor argument is an
element count matching `.length`. A `DataView` is different:

- it has no `.length` property, so `cur.length` is `undefined`;
- its third constructor argument is a byte length, not an element count.

`new DataView(buffer, byteOffset, undefined)` extends the view to the end of the
buffer, so a partial view's length is lost. (A DataView over the whole buffer
happens to work, because "to the end of the buffer" equals its byteLength.)

## Fix

Handle `DataView` explicitly and size it with `byteLength`:

```js
if (cur instanceof DataView) {
  return new DataView(cur.buffer.slice(), cur.byteOffset, cur.byteLength)
}
```

TypedArrays keep using the existing `.length`-based path.

## Test

Added tests (run for all four `rfdc` variants via the shared `types()` helper)
that clone a full-buffer DataView and a partial DataView, asserting `byteOffset`,
`byteLength`, the stored value, and independence from the source.

Counterfactual (revert the `copyBuffer` DataView branch): the partial-DataView
test fails with

```
not ok - byteLength preserved
  is(cloned.byteLength, view.byteLength, 'byteLength preserved')
```

(expected 8, got 12), while the full-buffer case still passes.

## Gate

- Tests: `node test/index.js` -> `1..144`, 144 ok, 0 failing, exit 0.
  The repository's `npm test` invokes the `tap` CLI, whose bundled `esm` loader
  crashes on load under Node 22 (`Function.prototype.apply was called on
  undefined`) independently of this change; running the test file directly
  executes the same assertions.
- Lint: `standard index.js test/index.js` -> exit 0.