chore: sync lockfile after rebase on main

Main picked up fumadocs-core/mdx bumps (#39) while this branch was
in flight; regenerate bun.lock against the current package.json so
bun install --frozen-lockfile passes on CI's merge-ref checkout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rsbh