ARCHITECTURE.md: initial iteration

[pedronis]

this document tries to give a high-level architecture and implementation overview

it's a starting point to navigate the code base and the why/philosophy of some parts of it

I worked with copilot to add references to code in the repo/documentation for the code

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 77.43%. Comparing base (69709ad) to head (92d1d41). :warning: Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #16271      +/-   ##
==========================================
- Coverage   77.49%   77.43%   -0.06%     
==========================================
  Files        1324     1334      +10     
  Lines      181813   181933     +120     
  Branches     2437     2437              
==========================================
- Hits       140895   140887       -8     
- Misses      32370    32497     +127     
- Partials     8548     8549       +1     

Flag	Coverage Δ
unittests	77.43% <ø> (-0.06%)	:arrow_down:

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[github-actions[bot]]

Fri Nov 21 15:43:21 UTC 2025

Spread tests skipped

[pedronis]

@miguelpires yes, I'm a bit busy, if you want to push changes for your suggestions + Graham's about dropping the bold in the section titles, except for the "run exec" stuff that needs its own discussion, please go ahead, thanks