Fix: Normalize Excel date cells to YYYY-MM-DD format (#243)

[vasanth2929]

Problem

When reading Excel files:

• Some date cells (t: "d") are parsed as raw numbers instead of Date.
• Display strings (cell.w) are inconsistent across environments (slashes vs dashes).
• This leads to mismatches between Excel UI and the xlsx parser.

Solution

• Normalizes date cells (t: "d") into the consistent ISO string format YYYY-MM-DD using XLSX.SSF.format.
• Skips non-cell metadata keys (!ref, !margins, etc).

Tests

• Verified against .xlsx files containing Date columns.
• Before: some dates rendered as raw numbers (e.g., 35994).
• After: all date cells consistently render as YYYY-MM-DD.

Fixes #243