Add index-aware Impose on lists and selections

[holonsoftFounder]

Summary

• Impose(member, i => value) and Impose(member, (i, item) => value) on ICollectionContext and ICollectionSequenceSelectionContext. The index is the position in the whole list (0 based), also inside First/Next and after Random: the generators now travel with their position (IndexedGenerator<TPoco>).
• Impose(member, item => value) on IObjectGenerator, backed by ObjectMemberSetFromFuncAction. The lambda runs after the configured sources and after every Impose registered before it, so it sees generated and imposed members.
• The classic Impose(member, value) and all other members are unchanged; the old selection-context constructor taking plain generators is kept.
• README example and release notes.

Test plan

• 16 new tests: unit tests with mocks for both contexts (position per item, selection only, position kept after Random, null factories), action tests for properties, fields, rejection of methods and nulls; integration for ids from the index, emails from generated members, earlier Impose visible, selection positions, Random keeps original positions, single generator, fields, mixing with the classic Impose
• dotnet build -c Release zero warnings
• 813 tests green per framework on net8, net9, net10 (Windows)
• Manual run of a file based app: ids 1..8, emails from names and index, First tags 0..2, Random tags at their original positions

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