docs(browser-db-sqlite-persistence): document multi-tab coordinator usage

[kevin-dp]

Summary

• The README described the package as Phase 7 single-tab browser wiring, which suggested multi-tab wasn't supported. In reality single-tab is just the default — passing a BrowserCollectionCoordinator via the coordinator option enables multi-tab coordination today (see examples/react/offline-transactions).
• Drop the internal phase reference, list BrowserCollectionCoordinator under the public API (it was already exported), split the quick start into single-tab and multi-tab sections, and add a worked multi-tab example with a brief explanation of how the coordinator uses Web Locks for leader election and BroadcastChannel for fan-out/RPC.

Test plan

• Render the README on GitHub and confirm both code blocks and the example link render correctly.

🤖 Generated with Claude Code

[pkg-pr-new]

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commit: 66ea51c