fix(v2): enforce connection lifecycle

[benbrandt]

Summary

• enforce initialize-first, exactly-once lifecycle semantics per v2 connection, including standalone initialize batches, cancellation, failure, and close behavior
• expose initialization snapshots, negotiated capability getters, and deferred onInitialized hooks while preserving onConnect timing
• preserve same-version initialize extensions after validation and add explicit prompt acceptance ordering with standalone prompt batches
• remove the dual-version example timer workaround
• keep the existing experimental v2 and underscore-extension API lane unchanged

Stack

This PR is stacked on #229 because it builds on the v2 method-contract types and response validation introduced there.

Verification

• npm run generate
• npm run check (40 files, 868 tests)
• focused v2 and protocol-router suite (55 tests)
• git diff --check