📝 docs: document that ImmutableMap | merges any Mapping

[nstarman]

Since #82, frozendict | other merges only a dict or frozendict, matching the Python 3.15 built-in. ImmutableMap.__or__ deliberately stays looser and merges any Mapping. This PR documents that difference and adds tests so it doesn't change by accident. There is no behaviour change.

Changes

• ImmutableMap.__or__ docstring: explains that ImmutableMap | other accepts any Mapping, while frozendict | other doesn't, and that the reflected other | ImmutableMap comes from frozendict and is not loosened.
• README ("Prefer the built-in frozendict" section): one paragraph on this difference, telling users switching to the built-in to wrap such mappings in dict(...).
• test_or now checks that:
  • ImmutableMap | Plain(...) merges (Plain is a Mapping with no |);
  • frozendict | Plain(...) raises TypeError;
  • Plain(...) | ImmutableMap raises TypeError.

Verification

• pytest: 135 passed on 3.10 and 3.13 (backport) and on 3.15.0 with jax 0.11.2 (built-in frozendict).
• prek run --all-files passes.

🤖 Generated with Claude Code

[codecov]

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Additional details and impacted files

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