refactor: clarify reconciliation input semantics

[SerDrozd]

Summary

Clarify the internal input model now that RowBridge supports both CSV and XLSX files, and lock in the current one-to-one resolution behavior with a regression test.

Changes

• rename the internal CsvTable model to InputTable
• keep CsvTable as a compatibility alias for existing imports
• remove the unused CsvInputError compatibility alias and development comment
• update internal type hints and architecture documentation
• add a regression test showing that one-to-one resolution is score-ordered greedy rather than a global assignment optimizer

Validation

• uv run ruff check .
• uv run mypy src tests
• uv run pytest - 59 passed
• git diff --check