[Aschenbath]

Copying a converter configured with union passthrough leaves spillover handling bound to the original converter. A hook registered on the copy works for A but is ignored for int | A; copy(forbid_extra_keys=True) can also accept extra keys inside that union.

Make the union hook factory an extended factory so dispatch supplies the converter generating the hook. Spillover then uses the copy's hooks and settings. Regression tests cover copy() and deepcopy(), warmed caches followed by hook registration, strict field validation, and a copied JSON converter loading a custom datetime format. The strategy docs and changelog describe the corrected behavior.

Validation:

• All 11 new regression scenarios fail before the fix and pass after it.
• Full suite on CPython 3.13 / Windows: 1,071 passed, 22 skipped, 15 xfailed.
• Ruff lint/format and Sphinx HTML with warnings treated as errors pass.
• The documentation doctests have two existing Windows/POSIX path-output mismatches, reproduced before the code change; 251 of 253 examples pass.

The full upstream CI workflow passes on this exact commit in the fork: all 10 jobs, covering CPython 3.10–3.15, PyPy 3.10, lint, package verification, and 100% combined coverage. The upstream Read the Docs build also passes.

Developed with AI assistance.

[Aschenbath]

Closing this one myself: I opened too many PRs here in a short time, which isn't fair to your review queue — sorry about that. I'm leaving #827 and #822 open. If this fix is something you'd want, I'm happy to reopen or rebase it later.