Add sys.monitoring coverage agent

[koltiradw]

Introduce a second coverage backend based on Python 3.12+'s sys.monitoring API (branch coverage map, no third-party deps), shipped as sys-monitoring/sitecustomize.py (formerly test.py).

Restructure the repo so each backend lives in its own directory with a dedicated README:

• coveragepy/: coverage.py/LCOV backend (formerly root sitecustomize.py)
• sys-monitoring/: new sys.monitoring branch-coverage backend

The root README now only gives a general overview and links to both backend READMEs.

[ThomasTNO]

Nice work @koltiradw. I am curious about performance differences in terms of throughput. How does your new approach compare to the old approach in terms of req/s?

[koltiradw]

@ThomasTNO hi!
I need a little time to take the measurements. I’ll be back with the results.

[koltiradw]

@ThomasTNO hi!
Results: https://github.com/koltiradw/dumb-bench-test

[ThomasTNO]

So, in summary, 15 req/s --> 408 req/s. Awesome!

@grebnetiew, willing to take a look as well?

[ThomasTNO]

One thing to have a look at is how much code we consider coverable. Can we also configure that for sys-monitoring?
e.g. coverage of original mode: coverage: 491/761 (64%), coverage of sys-monitoring: coverage: 91/65536 (0%)

Curious how the coverage differs and why (both total and hit blocks)

[ThomasTNO]

I guess the total for sys-monitoring is currently not a real total but rather an overly large map of 2^16.

[koltiradw]

You're right! The difference from coveragepy is that sys-monitoring tracks only branches, not every line. The bitmap size is currently 65536, since I can’t calculate in advance how many events there will be in total, but you can set a custom value via an environment variable.