fix: seeded mock generation now gives the same output on every run

[tx3stn]

Another mock determinism fix to plug the gaps still left over to make pb33f/doctor#119 changes deterministic.

Seeded mock generation now gives the same output on every run. Dates and times come from a fixed UTC instant instead of the clock, UUIDs come from the seed instead of crypto/rand, and XML children flattened with nodeType: none follow the schema's order instead of Go's random map order.

I built one spec that hits every branch of the renderer:

• every string format (date-time, date, time, email, hostname, ipv4, ipv6, uri, uri-reference, uuid, byte, password, binary, bigint, decimal), plus pattern, enum and length bounds
• every number format
• arrays, $ref, oneOf, anyOf, allOf and additionalProperties
• every XML node type: attribute, legacy attribute, text, none, wrapped and namespaced

version	lines that changed between runs
v0.41.5	13 of 194
these changes	0 of 194

Each of the 13 changed lines on v0.41.5 was one of the three fixes: dateTime, time, uuid (including a uuid nested inside allOf), or a flattened nodeType: none child that moved position. Nothing else changed.

Code audit:

• libopenapi renderer: no goroutines and no reflection over maps. With a seed set, it never reads the clock. The remaining map loops only copy values and don't draw random numbers. The render cache is rebuilt on every call, so one operation can't affect another's body.
• reggen draws everything from the generator it's given, and the renderer seeds that.
• frank has no clock, randomness or environment reads. Every loop goes over a slice or an ordered map, and the Go maps it uses are only for lookups.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (60b177e) to head (3f53470).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #654   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          309       309           
  Lines        39208     39213    +5     
=========================================
+ Hits         39208     39213    +5     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[daveshanley]

LGTM