Rebuild saved component relation indexes with their target entities

[antonsoo]

Loading a registry preserves component relation values but rebuilds their specific-target query index with the origin in place of the target:

import pickle
from tcod.ecs import Registry

world = Registry()
world["moon"].relation_components[int][world["planet"]] = 10
print([e.uid for e in world.Q.all_of(relations=[(int, world["planet"])])])
world = pickle.loads(pickle.dumps(world))
print([e.uid for e in world.Q.all_of(relations=[(int, world["planet"])])])
print([e.uid for e in world.Q.all_of(relations=[(int, world["moon"])])])

The current output is ['moon'], [], ['moon']. It should be ['moon'], ['moon'], []: loading creates neither a missing planet relation nor a self-relation.

Use target in _relations_lookup_from, matching _relations_lookup_add. This changes a derived lookup table, without changing the stored relation data or save format.

Regressions cover ordinary/named component keys, pickle protocols 2/4/5, specific and wildcard forward/reverse queries, the spurious self-match and unchanged tag relations. In 54 native saved orbital-scene configurations, all 2,934 incorrect specific-target query results are corrected. Every query matches its pre-save result; component values, reverse/tag controls and serialized byte hashes remain unchanged. The full pytest/doctest suite, mypy and project-pinned Ruff checks pass. Existing xpasses and the pytest parametrization warning are also present on main.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.15%. Comparing base (deeca21) to head (2d6ccf6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
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  Coverage   99.15%   99.15%           
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  Files           9        9           
  Lines         949      949           
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  Hits          941      941           
  Misses          8        8           

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[HexDecimal]

This was a critical fix. I've added this to the changelog myself.

I'll likely make a new release soon.