chore: rename drift-labs GitHub URLs to velocity-exchange

[ChesterSim]

Part of renaming the GitHub org drift-labs -> velocity-exchange.

DRAFT — do not merge until the velocity-exchange org (and, where applicable, the @velocity-exchange npm scope) exists; old URLs currently redirect.

URL forms changed

• github.com/drift-labs/ → github.com/velocity-exchange/ (Cargo.toml dep, Cargo.lock sources, README example, CI workflow)
• api.github.com/repos/drift-labs/ → api.github.com/repos/velocity-exchange/ (Dockerfile + CI: drift-ffi-sys release download)
• ghcr.io/drift-labs/ → ghcr.io/velocity-exchange/ (README Docker run example)
• drift-labs.github.io → velocity-exchange.github.io (README API docs links — v2-teacher, errors page)

Total substitutions: 12 across 5 files.

Files changed:

• .github/workflows/build.yml — drift-ffi-sys release download URL (1)
• Cargo.toml — drift-rs git dependency URL (1)
• Cargo.lock — drift-rs source URLs (×3) + jupiter-swap-api-client source URL (×1) (4)
• Dockerfile — drift-ffi-sys release download URL (1)
• README.md — ghcr.io image reference (1), github.io docs links (3), dlob-server example link (1) (5 substitutions)

Skipped (upstream-Drift historical refs)

None — all occurrences in this repo are operational references (dependency URLs, release asset downloads, container image paths, doc-site links for this gateway's own usage) rather than historical fork-origin statements.

Build note

drift-rs and jupiter-swap-api-client (a transitive dep of drift-rs) are sourced from GitHub git URLs. After the org rename, builds will fail until those repos exist under velocity-exchange. The Cargo.lock has been updated to reflect the new source URLs; cargo update may be needed once the repos are migrated.