Preserve proxy set operations and relation assignments

[antonsoo]

An in-place update of a relation set is written back to EntityRelations.__setitem__. That setter clears the targets before iterating its argument, so a live view of those same targets is now empty:

from tcod.ecs import Registry

world = Registry()
player = world["player"]
player.relation_tags_many["allies"] = {world["first"], world["second"]}
player.relation_tags_many["allies"] |= {world["third"]}
print(sorted(e.uid for e in player.relation_tags_many["allies"]))

Currently this prints []; it should print ['first', 'second', 'third']. Materialize the input iterable before clearing the old targets, which also handles lazy iterators over the same relation view.

The inherited binary set operations have a related constructor problem: their default factory tries to construct an entity-bound proxy from only an iterable. Provide a plain-set factory for both EntityTags and EntityRelationsMapping. Binary operations then return independent sets, and &= can compute its removal set without raising TypeError.

Regression checks cover all four in-place relation operators, cached queries, a lazy self-view, copying another relation key, non-mutating binary operations and tag intersection. Seven checks fail before the change; the separate-key assignment control already passes. The full pytest/doctest suite, mypy and project-pinned Ruff checks pass, with the same baseline xpasses and warning.

Nine native 80-turn party-selection runs are compared with plain Python sets and cached forward/reverse queries. The change removes 687 wrong turn states and 180 operator exceptions; all corrected histories match direct assignment. Nine direct-assignment controls retain identical histories across all 720 turns. This check validates game-state bookkeeping, without claiming performance or gameplay quality improvements.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (1696e6e) to head (3ba212d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #58   +/-   ##
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  Coverage   99.15%   99.16%           
=======================================
  Files           9        9           
  Lines         949      956    +7     
=======================================
+ Hits          941      948    +7     
  Misses          8        8           

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[HexDecimal]

I forget if I've heard about the _from_iterable classmethod before now. Luckily the entity attribute assignments are more for completeness than as a primary method of interacting with them. Still, it's really good to have this fix.

The code and tests are good, but what's fixed needs to be noted in the changelog. I can do that if you don't.