Align Mix Implementation with RFC

[AkshayaMani]

Opening this issue to track small improvements that bring the implementation more in line with the Mix Protocol RFC. These are based on recent discussions around the RFC feedback from @richard-ramos:

1. Introduce Randomized Delay
   Introduce a randomized delay before the initiator sends each Sphinx packet to the first hop. This is important to avoid metadata leakage when multiple Sphinx packets (e.g., due to fragmentation of a large message by an upstream protocol) are sent in a burst.

2. Add mixify Flag to Ping Examples
   Makes it clearer how upstream protocols can opt in/out of Mix routing. Helps clarify the entry layer handoff logic.

3. Add Standalone Spam Protection Example
   Add a self-contained PoW-based example (e.g., under examples/spam/) to demonstrate the pluggable nature of spam protection.

Only 1. is time-sensitive. 2. and 3. are not urgent but worth tracking to improve RFC compatibility.