Preserve subscript index reads during method inlining

[SAY-5]

Description

Inlining a function that assigns get_registry()[name] = value leaves name in the result instead of substituting the argument. Worder classifies the index as an assignment target when it sees a closing bracket before =.

Recognize that index as a read while preserving grouped and list assignment targets. Add regressions for subscript and slice indices, assignment/comparison operators, and the complete inline transformation.

Fixes #827

Validation

• The new assignment-classification and inline regressions fail on the original code and pass with this change; the focused run passes all 69 tests.
• Full suite on macOS ARM64, Python 3.13.9: 2,158 passed, 7 skipped, 5 xfailed. The run retains existing warnings, including a coverage warning for a deleted temporary fixture.
• Black 26.5.1: all 96 files selected by the repository configuration pass. git diff --check passes.

Checklist

• I have added tests that prove my fix is effective
• I have updated CHANGELOG.md

AI assistance: Codex assisted with the implementation, tests, review, and this description.