Fix Reduction.inames_set to return frozenset instead of mutable set

[Copilot]

Reduction.inames_set was a cached_property returning a mutable set, allowing callers to silently corrupt the cached value and affect all subsequent accesses.

expr = parse("sum(i, i**2)")
frozenset(expr.inames_set)          # frozenset({'i'})
expr.inames_set.add("bug")          # mutates the cache!
tuple(sorted(expr.inames_set))      # ('bug', 'i') — corrupted

Change

• Reduction.inames_set now returns frozenset[str] instead of set[str], making the cached value immutable. All existing call sites using set operations (&, -, etc.) are unaffected since frozenset supports them.

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[Copilot]

Pull request overview

This PR fixes a mutability hazard in Reduction.inames_set by ensuring the cached value cannot be modified by callers, preventing silent corruption of subsequent accesses.

Changes:

• Change Reduction.inames_set (a cached_property) to return frozenset[str] instead of set[str].

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