@unthrown/vitest: add toBeDefectWith(cause) to complete the matcher set

[btravers]

@unthrown/vitest ships six matchers. The Defect channel is the only one
without a value assertion:

channel	existence	value
Ok	toBeOk	toBeOkWith(value)
Err	toBeErr	toBeErrWith(error), toBeErrTagged(tag, payload?)
Defect	toBeDefect	—

Asserting what caused a defect currently takes a two-step, and the narrowing
step only works on a synchronous Result — on an AsyncResult the value has to
be awaited separately, which is exactly what the matchers exist to avoid:

const r = await someOperation();
expect(r).toBeDefect();
if (r.isDefect()) expect(r.cause).toBeInstanceOf(TypeError);

Proposal

Add toBeDefectWith(cause), mirroring toBeErrWith
(packages/vitest/src/index.ts:270) exactly:

function toBeDefectWith(this: MatcherState, received: unknown, expected: unknown): MatcherResult {
  const { stringify } = this.utils;
  const { equals } = this;
  return settle("toBeDefectWith", this, received, (result) => {
    const pass = isDefect(result) && equals(result.cause, expected);
    …
  });
}

Going through settle means it inherits the thenable handling and the
forgotten-await net for free, like every other matcher.

Usage:

await expect(asyncResult).toBeDefectWith(expect.any(TypeError));
expect(result).toBeDefectWith(theOriginalCause);

expected is unknown, matching the channel: a defect's cause is unknown by
design, so there is no tighter type to give it and no tag-aware variant to add
(the toBeErrTagged counterpart has no meaning here).

Checklist

• expect.extend registration and the raw-function export (index.ts:341-343).
• The UnthrownMatchers type (index.ts:397-399).
• CLAUDE.md — the vitest package description says "the six raw matcher
  functions" and the status section enumerates them.
• docs/how-to/test-with-vitest.md and the skills/unthrown/ copy, in the same
  PR (the skill is hand-maintained and drifts).
• A changeset.