Add TracingChannel support for APM instrumentation

[sergical]

Summary

• Adds native diagnostics_channel TracingChannel instrumentation to Remix 3's three subsystems
• remix:request — traces middleware and route handler execution in fetch-router
• remix:render — traces server-side rendering in ui/server
• remix:asset — traces on-demand script/style compilation in assets
• Zero cost when no subscribers are registered (hasSubscribers gating)
• Includes a demo app (demos/tracing-channels) that subscribes to all three channels and logs spans with timing

This enables APM tools (Sentry, OpenTelemetry, Datadog, etc.) to subscribe to structured lifecycle events without monkey-patching. Since Remix 3 is a closure-based, ESM-only framework, monkey-patching would be especially fragile — TracingChannel provides a stable, first-party surface from day one.

Traced request flow

remix:request (type: handler) — GET /
  remix:render               — renderToStream
    remix:request (type: handler) — GET / (frame sub-request)

remix:request (type: handler) — GET /assets/*path
  remix:asset                — script compilation

Demo output

✓ TracingChannel subscribers active (remix:request, remix:render, remix:asset)
→ handler GET /
  → render http://localhost:44200/
  ← handler GET / 1.86ms
← render http://localhost:44200/ 3.91ms

Test plan

• All 77 existing fetch-router tests pass
• All three packages type-check clean (tsc --noEmit)
• Demo app starts, serves requests, and logs tracing spans
• No changes to existing behavior when no subscribers are registered
• Verify with a real Sentry SDK subscriber (next step: @sentry/remix)

🤖 Generated with Claude Code