[codex] Harden Summon streaming

[nahiyankhan]

Summary

• Add a backpressure-aware createProtocolLineWriter public server helper and wire the demo generation route through it.
• Add cancelOnStop support to consumeSurfaceStream() so stopped streams cancel by default.
• Update adoption docs and public package export/smoke checks for the hardened streaming path.

Validation

• pnpm --filter @summon-internal/server test
• pnpm --filter @summon-internal/host test
• pnpm --filter @summon-internal/engine test
• pnpm typecheck
• pnpm build:impl
• pnpm build:public-core
• pnpm check:public-api
• pnpm smoke:public-packages (passed after transient npm registry metadata warnings)

Notes

This is stacked on #14 (codex/terminology-layering) so the PR diff stays focused on streaming hardening.