refactor(core): replace getAddress with placeholder-based address injection

[aminevg]

Summary

• Removes getAddress callback from MightyDevOptions — the address is now passed per-request via MightyRenderDevRequest.address
• Internally uses a hardcoded placeholder (http://mighty-placeholder.test) during setup/render, then replaces it with the real address via replaceAll on the final HTML
• Simplifies the Hono adapter by removing mutable rootAddress state and computing the address fresh per-request
• Inlines rewriteFsUrls into setup.ts and deletes the utility file

Test plan

• pnpm ci:biome passes
• pnpm typecheck passes in both packages/core and packages/hono
• pnpm test — all 47 tests pass