…eet layout

Add style support for Calamine xlsx files.

Public API:
- `Xlsx::worksheet_style(sheet)` returns a row x col grid of cell styles
  using run-length encoding for memory efficiency on large workbooks.
- `Xlsx::worksheet_layout(sheet)` returns column widths and row heights.

Style types (in `src/style.rs`):
- `Style` with optional Font / Fill / Borders / Alignment / NumberFormat
  / Protection.
- `Color` with theme + tint resolution and indexed-color fallback.
- `RichText` / `TextRun` for cells with mixed inline formatting.
- `StyleRange` with RLE storage and a `cells()` iterator.

Parser in `src/xlsx/style_parser.rs` handles fonts (bold / italic /
underline / strikethrough / sz / color), fills, borders (with color and
style per side), number formats (built-in + custom format codes),
alignment (horizontal / vertical / wrap / indent / shrink / text
rotation incl. stacked), protection (locked default per OOXML), theme
colors with tint, and sysClr lastClr fallback.

Shared-string reader now decodes rich text runs and preserves their
formatting, while also handling plain text that precedes rich runs
(consistent with upstream PR tafia#637).

Includes benchmarks in `benches/style.rs` and test fixtures
(styles.xlsx, borders.xlsx, EMSI_JobChange_UK.xlsx,
problematic_formats.xlsx, styles_1M.xlsx) covering the various code
paths.

Co-authored-by: Cursor <cursoragent@cursor.com>