CI: auto-generate wit-bindgen fixtures instead of checking in binaries

[avrabe]

Current state

The wit-bindgen test fixtures (~6.6 MB each) are checked into the repo as binary blobs under tests/wit_bindgen/fixtures/. Tests skip gracefully when fixtures are absent.

Proposal

Add a CI step (or Makefile target) that generates fixtures from the wit-bindgen source tree, so:

• Binary blobs don't bloat the repo
• Fixtures stay in sync with wit-bindgen updates
• Contributors can regenerate locally with a single command

Approach options

1. Git submodule — pin wit-bindgen at a specific commit, generate in CI
2. Download from release — fetch pre-built fixtures from wit-bindgen releases
3. Makefile/script — make fixtures that clones wit-bindgen, builds, and copies

Tests already handle missing fixtures via fixtures_available() guard, so this is backwards-compatible.