staslib: let stacd work out what it was managing instead of remembering

[martin-belanger]

stacd wrote the TIDs of the I/O controllers it managed to a pickle in $RUNTIME_DIRECTORY and, on startup, re-created a controller for each one that still had a connection in the kernel. That intersection is the tell: the kernel already knew what was connected, and the pickle only narrowed it to what had been ours. The registry answers that question now - libnvme records owner=stas on the controllers it connects for us, and claim() marks the ones we borrow - so nothing is left for stacd to remember.

Ask the kernel what is connected and the registry who owns each one. This is more robust than the file it replaces: it survives a stacd that was killed before it could write, and it cannot go stale, because the registry entry is keyed by device and goes away with it.

Adoption requires an entry naming us, rather than merely one not naming somebody else. protected() lets an unowned connection through on purpose - nobody has claimed it, so we may - but that is the wrong default here. Starting with no memory of what we were doing, an unowned connection is one we did not make: it predates the registry, or belongs to somebody who never registered it.

stafd keeps its own file. It holds the cached discovery log pages and each controller's origin, neither of which the registry can supply, so the pickle machinery moves from ServiceABC down into Staf. stacd no longer writes to $RUNTIME_DIRECTORY at all, so its unit file no longer asks for one.

Udev.get_tid() comes back, unchanged, from 27705e8 where it was dropped for want of callers.

[codecov]

Codecov Report

❌ Patch coverage is 43.24324% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.93%. Comparing base (d3c6e77) to head (9340e92).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
staslib/service.py	43.47%	13 Missing ⚠️
staslib/udev.py	33.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
+ Coverage   68.71%   68.93%   +0.21%     
==========================================
  Files          16       16              
  Lines        2730     2739       +9     
==========================================
+ Hits         1876     1888      +12     
+ Misses        854      851       -3     

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