Add .xlsb workbook support through a separate reader

[kingkillery]

Problem

The encoder uses openpyxl, which does not support .xlsb files. During smoke testing, Clawbacks Working.xlsb had to be skipped.

Why this matters

Real spreadsheet workflows often include binary Excel workbooks. Skipping .xlsb limits coverage for operational data and makes the encoder less useful on real user files.

Proposed work

• Add an optional .xlsb reader path, likely behind an extra dependency such as pyxlsb.
• Normalize .xlsb sheets into the same internal worksheet-like representation used by the encoder.
• Clearly document unsupported features if formulas, styles, merged cells, or number formats cannot be fully recovered.
• Keep .xlsx behavior unchanged.

Acceptance criteria

• .xlsb inputs fail with an actionable message when the optional dependency is absent.
• With the optional dependency installed, simple .xlsb sheets can be encoded.
• Tests cover dependency-missing and dependency-present paths where practical.
• Documentation states any fidelity limitations versus .xlsx.

[kingkillery]

@codex