test API: portable toMatchSnapshot (snapshots) across bun/node/browser

[brainkim]

Summary

@b9g/libuild/test unifies expect across runtimes, but snapshot matching isn't portable. Under bun, expect is bun:test's (has toMatchSnapshot); under node it's the expect package, which has no snapshot support without a host runner. So expect(x).toMatchSnapshot() passes on -p bun and fails on -p node with:

TypeError: expect(...).toMatchSnapshot is not a function

It also doesn't typecheck: the unified expect type is the union of the two, and the union has no toMatchSnapshot.

Why it matters

Snapshots are the natural way to test large rendered-output blobs (for termdom, multi-line terminal renders). Right now a project has to either drop snapshots or keep them bun-only, which defeats the point of a cross-platform runner.

Proposed

Provide a portable snapshot matcher in the libuild test layer, independent of the underlying runner:

• expect(value).toMatchSnapshot() serializes value, stores it in a .snap file next to the test (or a __snapshots__ dir), and compares on subsequent runs.
• An update mode — env var (LIBUILD_UPDATE_SNAPSHOTS=1) and/or a CLI flag (--update-snapshots / -u).
• Same behavior and same snapshot files across bun, node, and browser platforms, so a snapshot written on one platform verifies on the others.
• Include toMatchSnapshot in the exported expect type so consumers typecheck.

This is the same "unify the runtimes at the libuild layer" shape as the externalize (#12) and setup-file (#13) fixes.

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Filed from termdom: ~11 tests across ~7 files snapshot rendered terminal output and currently pass on bun, fail on node.