chore: require preact-render-to-string >=6.7.0 as the optional peer

[JoviDeCroock]

Note

This PR was authored by an LLM (Claude Fable 5.1) during the v11 release-readiness audit; Jovi reviewed the change and the tests.

Summary

Bumps the optional preact-render-to-string peer range from >=5 to >=6.7.0.

Preact 11 is ESM-only, so preact/compat/server is now imported through compat/server.mjs, which does import … from 'preact-render-to-string/stream-node'. In preact-render-to-string ≤ 6.6 the import condition of that subpath points at a dist/stream-node.mjs that is not published, so the import fails with ERR_MODULE_NOT_FOUND at startup. 6.7.0 publishes the entry (dist/stream/node/index.mjs) and its peer range already allows Preact 11.

The devDependency is left at 6.5.0 for now: 6.7.0's dist/stream-node.d.ts imports WritableStream from node:stream, which no @types/node exports, and that fails this repo's tsc -p jsconfig-lint.json. That is a one-line upstream fix (import Writable); the devDependency can follow once it ships.

[coveralls]

coverage: 99.534%. remained the same — JoviDeCroock/rts-peer-6-7 into main