feat: use rng in mockstream

[nstarman]

Ping @jnibauer. I think there are further improvements to be made, e.g. I think we can iterate over the rng rather than evolve it within the scan, but this is for a followup.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.66%. Comparing base (c972652) to head (a39517c).
Report is 62 commits behind head on main.

Files	Patch %	Lines
...amics/_dynamics/mockstream/mockstream_generator.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
+ Coverage   88.44%   89.66%   +1.22%     
==========================================
  Files          42       54      +12     
  Lines        1324     1829     +505     
==========================================
+ Hits         1171     1640     +469     
- Misses        153      189      +36     

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