feat: Medium effort - fidelity divergences docs, ID rendering tests, Substitution promotion

[csharpfritz]

Summary

Addresses the medium-effort items from the component audit:

Known Fidelity Divergences Documentation

• New doc: docs/MigrationGuides/KnownFidelityDivergences.md
• Documents ListView DOM divergence (High), Calendar sub-element IDs (Medium), Label element selection (Low), FormView non-table path (Low)
• Includes ID rendering coverage summary and CSS/JS targeting recommendations
• Added to Migration nav section in mkdocs.yml

ID Rendering Test Coverage

• Discovery: 7 of 8 data controls already render id=ClientID (GridView, DetailsView, FormView, DataList, DataGrid, DropDownList, HiddenField). Only ListView cannot (uses user-provided LayoutTemplate).
• Added 5 new bUnit tests (DataList: 3 tests, HiddenField: 2 tests)
• Updated audit to mark Priority Initial ListView control with data read features #2 as DONE

Substitution Promotion

• Updated tracked-components.json: Substitution status changed from Deferred to active
• Component was already fully implemented with docs and sample page

Test Results

• 2,475 bUnit tests passing (5 new)
• MkDocs strict build passes cleanly