Support static pictures inside cells with XLSX roundtrip (#61)

docs/cell-pictures.md

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+# Pictures inside cells
+
+Choose **Insert → Pictures → Place in Cell**, select a PNG or JPEG file, and insert it into the
+active cell. The picture replaces that cell's contents while keeping its formatting. Undo
+restores the previous contents.
+
+The picture fits inside the cell without stretching. It follows row and column resizing,
+sorting, filtering, insertion/deletion and GridCraft's internal copy/cut/paste. Clearing
+contents removes it; clearing or pasting only formats keeps it. Save as XLSX to preserve the
+embedded image and its alt text, or export to PDF for a rendered copy.
+
+**Place over Cells** retains the existing floating-picture behavior.
+
+The command API accepts the same placement:
+
+```json
+{"command":"insert.picture","params":{"path":"picture.png","placement":"cell","at":"B2","alt":"Product photo"}}
+```
+
+This initial feature handles static pictures. Pasting an image from the operating system's
+clipboard, converting between floating and cell pictures, and the `IMAGE()` function remain
+separate work. Formula references and arithmetic do not carry pictures: the calculation
+engine uses a `#VALUE!` fallback, including when a picture is referenced from another cell.
+Text-only exports also use that fallback. The picture cell still counts as occupied and
+blocks a formula's spill range.
+
+XLSX stores these pictures as rich values. Readers without support for that format may show
+`#VALUE!` instead of the image. GridCraft reads embedded PNG/JPEG rich-value images; unsupported
+rich content, oversized or damaged picture metadata, and broken image references produce import
+warnings while the workbook opens with scalar fallbacks. It does not fetch linked
+images from the internet.