feat: add Fastify v4/v5 dual compatibility in package and CI

[denbon05]

Summary

• Add explicit dual Fastify peer support by changing peerDependencies.fastify to ^4.10.0 || ^5.0.0.
• Update plugin metadata in lib/index.ts (fastify-plugin compatibility range) to the same dual-major constraint.
• Raise supported Node engine to >=18.x.
• Upgrade dev Fastify to v5 and refresh lockfile.
• Expand CI matrix to test both Fastify 4.x and 5.x across Node LTS versions, excluding unsupported combo (node 18.x + fastify 5.x).

Why

• Keep compatibility for existing users on Fastify v4 while enabling users migrating to v5.
• Address dependency/security pressure on Fastify v4 without forcing an immediate hard cutover for all consumers.
• Continuously verify compatibility in CI rather than relying on a single dev dependency version.

Test plan

• npm run build
• npm run lint
• npm test with Fastify v5
• npm install --no-save fastify@4.x && npm test (manual compatibility check)
• CI matrix now validates both Fastify majors on PRs