Share browser hibernation host helpers

[MiguelsPizza]

Why

Rook hibernation integration #146 proved that two pieces of the reference embedder were being copied into downstream hosts: the in-memory hibernation mirror and the browser Request/Response upgrade accommodation. Those copies contain lifecycle-sensitive behavior, so the package should own and test them once.

This follows the hibernatable WebSocket runtime added in do-runtime #37 and matches Cloudflare documentation that acceptWebSocket() keeps clients connected while the Durable Object is evicted and reconstructed: https://developers.cloudflare.com/durable-objects/best-practices/websockets/

What changed

• export HibernationMirror from the package root, including copied metadata snapshots and rehydration seeding
• add @mcp-b/do-runtime/browser for the clone-safe Request marker and status-101 Response socket
• switch the Node/browser conformance hosts and extension example to those published APIs
• delete the two example-only shim copies
• add a patch changeset so Rook can consume this without a local link or vendor patch

Verification

• unit: 852/852
• conformance: workerd 69/69, Node 69/69, browser 77/77
• typecheck: package, conformance, and all example projects
• package: publint, Are the Types Wrong, and 69-file smoke test
• extension and vibe-platform E2Es: green
• workerd oracle pin: green

Downstream

Once the Changesets release publishes, Rook can delete its local WorkerHibernationMirror and browser upgrade shim and import these owners directly.