Excel anchors a floating object to the grid in one of three ways, stored in the
file as the drawing anchor's `editAs`: "move and size with cells" (twoCellAnchor,
the default), "move but don't size with cells" (oneCellAnchor / editAs="oneCell")
and "don't move or size with cells" (editAs="absolute"). GridCraft had no such
concept: xlsx import dropped `editAs`, export always wrote a plain twoCellAnchor,
and sorting/filtering never moved an object — so pictures stayed put when their
rows were reordered.
Model: add `AnchorMode` on `Anchor` (serde default = MoveAndSize, so existing
documents round-trip unchanged).
xlsx: read the anchor kind and `editAs` into the mode; write it back out
(editAs="oneCell"/"absolute" as Excel does).
Engine: sorting now moves objects anchored inside the sorted body with their
lines, re-fitting "move and size" objects, keeping "move but don't size"
objects' size, and leaving absolute ones pinned. Structural insert/delete
honours the same rule (previously it moved every object's cell unconditionally
and ignored shapes). New `object.setAnchorMode` command sets an object's mode;
`arrange.selectionPane` reports it.
Tests: sort moves images with their rows; absolute objects stay put across a
sort and a row delete; move-only keeps its size; the mode survives an xlsx
round-trip; the drawing fixture parses all three anchor kinds.
cbc2430f4f