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Fix unnecessary parentheses in indexed assignment RHS (#5095)

* Fix unnecessary parentheses in indexed assignment RHS (#4349)

When an assignment target contains brackets (e.g. indexed access like
`x[key] = expr`), Black would wrap the right-hand side expression in
unnecessary parentheses when the line was too long. For example:

    dictionary["key"][idx] = (10 - 5)

This happened because `can_omit_invisible_parens` returned False for
simple expressions with operators (like `10 - 5`) that don't start or
end with brackets. The function was too conservative: it didn't consider
that the line could instead be split at the LHS brackets, producing:

    dictionary["key"][
        idx
    ] = 10 - 5

The fix allows omitting optional parens when the RHS body is short
enough and the LHS contains brackets that can absorb the split. The
downstream `_prefer_split_rhs_oop_over_rhs` still decides whether the
resulting layout is actually better.

Fixes #4349.

* Gate indexed assignment parens fix behind preview mode and add CHANGES.md

Move the fix for unnecessary parentheses in indexed assignment RHS
behind a Preview flag to avoid unintentional stable style changes.
Test cases moved to a preview-specific test file.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Address review feedback: make mode required, use AST-based length check, add docs

- Make `mode` parameter required in `can_omit_invisible_parens` to prevent
  accidental omission at call sites (fixes flake8 B008)
- Replace `str_width(str(line))` with `enumerate_with_length()` for an
  AST-only body length calculation (avoids expensive string conversion)
- Document `fix_unnecessary_parens_in_indexed_assignment` in
  `docs/the_black_code_style/future_style.md` (fixes test_feature_lists_are_up_to_date)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Regenerate schema to include fix_unnecessary_parens_in_indexed_assignment

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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