Preserve original names in external from-imports

[ryanchou1994]

Renaming a project binding imported from an external module currently rewrites the external name too. In #831, renaming the compatibility module's izip produces an import of a name that itertools does not provide.

Preserve the source name of an external from import and add or update its local alias instead. Imports of definitions within the project still rename together. Import positions come from the existing AST offset adapter, and module resolution is limited to matching names so unrelated invalid imports do not block the rename.

Fixes #831.

Tests cover builtin and external Python modules, existing aliases, conditional cross-module bindings, multiline/Unicode source, and an unrelated invalid import. This is limited to from imports; the existing behavior for module-qualified external references is unchanged.

• Added regression tests (5 fail on the unchanged base; 2 are behavior controls)
• Updated CHANGELOG.md

Validation: 104 rename tests pass. Full suites pass on macOS/Python 3.14 (2,155 passed, 7 skipped, 5 xfailed) and Linux/Python 3.12 (2,151 passed, 11 skipped, 5 xfailed); Black passes.

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