Remove maps fastify plugin from @comapeo/core to drop iOS-only bundler work

Context

Milestone-doc nice-to-have: "Remove maps fastify plugin from core, so that we can remove extra build work that's done for iOS."

The map server (fastify) currently lives in core and forces extra backend-bundler handling on iOS (undici/polywasm/fetch shims — see backend/index.ios.js, backend/rolldown.config.ts). If the maps plugin moves out of core, that iOS-specific bundling could be simplified or removed.

Scope (to refine)

• Confirm what the iOS bundle pulls in solely because of the maps fastify plugin.
• Coordinate with @comapeo/core to extract the plugin.
• Simplify rolldown.config.ts / iOS fetch shims once the dependency is gone.

Note: the bundler has since moved rollup → rolldown (#94); re-evaluate the actual remaining cost before doing this.

Start with backend/index.ios.js and backend/rolldown.config.ts, then trace which iOS bundle dependencies come solely from the maps Fastify plugin. Coordinate with @comapeo/core on extracting the plugin and re-evaluate the remaining bundler cost after the Rollup-to-Rolldown move. Done means the plugin is no longer in core and unnecessary iOS bundler or fetch-shim work is simplified or removed.