Copy component relations between distinct keys on the same entity

[antonsoo]

Assigning a component relation view to another key on the same entity silently returns without copying:

from tcod.ecs import Registry

world = Registry()
mage, enemy = world["mage"], world["enemy"]
mage.relation_components["fire", int][enemy] = 5
mage.relation_components["poison", int] = mage.relation_components["fire", int]
print(mage.relation_components["poison", int].get(enemy))

This currently prints None; it should print 5. The self-view shortcut checks the owner entity but ignores the component key. Require both to match before treating assignment as a no-op, then reuse the existing replacement path for different keys.

Regression checks cover ordinary/named source keys, removing old destination targets, source preservation and an unchanged copy from another entity. On 36 native combat-effect map configurations, the change corrects 522 wrong target values and the corresponding 522 cached query results. All 18 different-owner controls and every source map remain exactly unchanged. The full pytest/doctest suite, mypy and project-pinned Ruff checks pass, retaining the same baseline xpasses and warning.

This changes only the self-view shortcut. It does not change copying depth, relation storage or the save format.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (a1d4980) to head (e88e94c).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #59   +/-   ##
=======================================
  Coverage   99.16%   99.16%           
=======================================
  Files           9        9           
  Lines         956      956           
=======================================
  Hits          948      948           
  Misses          8        8           

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[HexDecimal]

I've added the missing changelog note. Otherwise this has been another good fix.