chore: import shaal/VectorVroom as submodule under examples/

[ruvnet]

Summary

Imports shaal/VectorVroom as a git submodule under examples/vectorvroom. VectorVroom is a browser-based genetic-algorithm car racer that uses RuVector's WASM build for a "cross-track vector-memory bridge" — effectively a downstream demo of the ecosystem running entirely in the browser with no build step.

• Repo: https://github.com/shaal/VectorVroom
• Homepage: https://vectorvroom.shaal.dev
• Size: 3.4 MiB Language: JavaScript Stars: 8
• Pinned SHA: `4c2527b4526ccb8960cd13e3d9e1802d958dca60` (upstream "fix(ab-mode): sync baseline worker…")

What lands in this PR

• `.gitmodules` — new file, declares the submodule
• `examples/vectorvroom` — new gitlink entry (mode 160000) pointing at the pinned SHA

Consumer notes

Fresh clones get an empty `examples/vectorvroom/` directory unless they run:

```bash
git submodule update --init examples/vectorvroom
```

That's the standard submodule idiom — nothing new for contributors already familiar with it. The rest of the workspace is unaffected: `examples/*` is already excluded from the root `[workspace] members` list, so `cargo check` / `cargo test` / `cargo build` at the root don't touch the submodule.

To update the submodule to upstream `main` later:

```bash
git submodule update --remote examples/vectorvroom
git commit -am "chore(vectorvroom): bump to upstream main"
```

Licensing note

shaal/VectorVroom currently declares no license (GitHub's API returns `license: null`). This matters if we ever embed or redistribute its code in a RuVector release artifact. As a pure submodule pointer we're only vendoring a clone URL + commit SHA — no VectorVroom source enters the ruvector tree itself, so this import is fine as-is. Worth revisiting if:

• We ship a bundled release that includes VectorVroom assets
• We want to link to VectorVroom from ruvector docs as "officially part of the ecosystem"

Either of those would need a license clarification with the upstream author first.

Test plan

• `git submodule update --init examples/vectorvroom` succeeds from a fresh clone
• `cargo check` at the root remains unaffected
• No CI workflows inadvertently try to build inside `examples/vectorvroom/`

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