feat(sheet): expose worksheet name and used-range origin on Table

[alexiskowalski]

Summary

Fixes #10 by preserving spreadsheet worksheet identity and used-range origin on Table.

to_document() cropped Excel ranges to a 0-based grid and, for single-sheet workbooks, dropped the sheet name entirely. Callers could not tell that grid [0][0] came from C3 on Data Sheet.

Changes

• Add optional sheet_name, source_row, and source_col on Table (absolute 0-based origin of the used range).
• Set those fields in the Excel frontend for every sheet (single- and multi-sheet).
• Set sheet_name for ODS spreadsheet tables when the table has a name.
• Expose the fields through Python / Node / WASM bindings.
• Markdown output is unchanged (provenance stays on the document model).

Test plan

• Unit test: single-sheet workbook with a value at C3 asserts sheet_name == "Data Sheet" and source_row/col == 2.
• cargo test --locked formats::sheet
• CI