feat(plugin-table): distribute a pasted table fragment over the target cells

Pasting a copied rectangle into a table dropped the whole fragment
into one cell (not even schema-legal as a nested table). It now
distributes cell-per-cell from the anchor, the spreadsheet convention:
the target rectangle's top-left, or the caret's cell. A larger target
rectangle only provides the anchor; the extra target cells stay
untouched rather than clearing.

The interception sits on `clipboard.paste`, before core's
expanded-selection `delete` decomposition, because the target
rectangle must survive to define the distribution. Detection is not
name-sniffing alone: the clipboard's `application/x-portable-text`
data goes through the same converter the editor's own paste uses,
validating the fragment against this editor's schema down into the
cells' blocks, and the result must be a single canonical table.
Everything else falls through to the editor's paste handling, keeping
the replace-rectangle behavior for non-table content.

Two hardening layers the converter cannot provide: the parser
preserves existing keys, so every pasted block is re-keyed with
markDef references remapped (pasting a column next to itself cannot
duplicate keys), and since the converter is deliberately lenient
(`validateFields: false`) while the distribution applies content
through raw `insert` primitives, cell content is filtered against the
cell's own sub-schema: block types the cell does not declare never
reach the document, pinned by a rogue-fragment test.

A fragment extending past the table's edges grows rows and columns
with fully-constructed nodes, padding off-fragment positions with
empty cells and extending the positional `alignment` array with
`null`s. All raises form one undo step, and the pasted rectangle
becomes the selection.

Author: christianhg