y-partyserver: extract withYjs mixin

[threepointone]

Extract Yjs functionality into a withYjs mixin that can be applied to any Server subclass.

Changes

• withYjs(Base) — new exported mixin function. All Yjs logic (document management, sync protocol, awareness tracking, persistence, custom messages) lives inside this mixin. Can be applied to any class extending Server:

  import { Server } from "partyserver";
  import { withYjs } from "y-partyserver";
  
  class MyCustomYjsServer extends withYjs(Server) {
    // has full Yjs functionality
  }

• YServer — now defined as withYjs(Server), fully backward-compatible
• YjsInstance / YjsStatic — new exported interfaces describing the mixin's instance and static members

Verification

• Type checking passes
• Build passes (ESM + CJS + .d.ts)
• All 23 existing tests pass with no changes

[changeset-bot]

🦋 Changeset detected

Latest commit: c0d0112

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
y-partyserver	Patch

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partyfn

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partyserver

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partysocket

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partysub

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partysync

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partytracks

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partywhen

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y-partyserver

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commit: c0d0112