Release v2.21.0

What's Changed

• change copies to slackbot by @ignaciojimenezr in #2906
• feat(host-compat): lanes + conformance gate + Compatibility destination by @chelojimenez in #2922
• feat(evals): Export traces button (OTLP/OpenInference) on the run detail page by @chelojimenez in #2921
• feat(host-compat): Phase 2 — multi-server compatibility matrix by @chelojimenez in #2926
• test(host-compat): restore real timers in afterEach (no fake-timer leak) by @chelojimenez in #2924
• feat(host-compat): Phase 3 (1/2) — live "observed" widget render by @chelojimenez in #2930
• feat(server): land caniuse.dev on the host-compare showcase by @chelojimenez in #2911
• feat(sdk): host-compat engine in @mcpjam/sdk/host-compat (Phase A) by @chelojimenez in #2932
• refactor(sdk): solidify host-compat abstraction (narrow detection + semantic findings) by @chelojimenez in #2934
• fix @modelcontextprotocol/client to stable version by @ignaciojimenezr in #2939
• changesets by @ignaciojimenezr in #2940

Full Changelog: v2.20.0...v2.21.0