Add deterministic capability exposure ordering in initialization responses

[davidahmann]

Problem
Capability ordering in initialization responses can vary, producing snapshot churn and non-reproducible handshakes for clients/tests.

Why now
Reference servers should provide stable protocol surfaces and deterministic initialization behavior.

Expected behavior
Equivalent configurations produce byte-stable capability ordering in initialization responses.

Acceptance criteria

• Capability exposure ordering is deterministic across runs.
• Existing capability semantics unchanged.
• Tests assert stable init response ordering for equivalent config.

Evidence packet

• Commit under test: 04cce79
• Runtime environment: macOS Darwin 25.3.0 arm64, Node 22.19.0
• Minimal repro:
  1. Start server repeatedly with same config.
  2. Capture init response capability sections.
• Expected: stable deterministic ordering.
• Actual: ordering may vary and cause churn.