[antonsoo]

Replacing a prefab's component relations from an instance's inherited mapping silently loses values: clearing the destination also changes the live source before its items are read.

import tcod.ecs
world = tcod.ecs.Registry()
prefab, target = world['prefab'], world['target']
prefab.relation_components[int][target] = 3
instance = prefab.instantiate()
prefab.relation_components[int] = instance.relation_components[int]
print(dict(prefab.relation_components[int]))
# before: {}; after: {target: 3}

Snapshot the input pairs before clearing the destination, matching the existing approach for tag-relation assignment. Component objects are retained by reference; this adds an O(n) temporary tuple of pairs. The same-entity/same-key no-op guard stays unchanged. Add regressions for named/unnamed keys, one/three inheritance levels, overrides and warmed queries, plus a changelog entry.

Validation on current main 07ba4b8 (including #61):

• Eight new cases fail before and pass afterward. Full suite on Python 3.10 and 3.12: 96 passed, two existing xpasses and one existing warning; baseline is 88 passed with the same xpasses/warning.
• In 48 executed prefab-edit/save/load game scenarios, 784 lost relation entries, 23,520 missing actor-query memberships and 7,680 wrong target decisions become zero. All 48 frozen baseline archive hashes and all copied-dictionary control histories remain exact; both Python versions agree. This is a constructed game-system workload, not a claim about a released game.
• Full mypy passes; applicable pre-commit checks, Ruff lint and formatting pass.

This is separate from #59's distinct-key no-op guard and #61's overlapping tag/component query indexes: the input here inherits data from the destination being replaced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.17%. Comparing base (07ba4b8) to head (a591383).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #62   +/-   ##
=======================================
  Coverage   99.17%   99.17%           
=======================================
  Files           9        9           
  Lines         965      966    +1     
=======================================
+ Hits          957      958    +1     
  Misses          8        8           

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[HexDecimal]

Treating these as collections has been awkward with how Python references everything. At least the solution to these issues is often to simply make a copy of the mutable state before processing.

Thanks for more of these fixes and tests.

[HexDecimal]

Extra newline here.

[antonsoo]

Removed the extra blank line. Thanks for reviewing.