Closes#11682.
Fixes two bugs that caused a simple table to be read as a cascade of
nested tables:
* `\plain` reset the entire property record (including the in-table
flag) via `const def`. When a cell paragraph used `\plain` after
`\intbl`, the in-table flag was cleared, so the next `\cell` closed
the partially-built table and embedded it inside a new cell. Per the
RTF spec, `\plain` should reset only character formatting, so it now
preserves paragraph/context properties (in-table, list level, outline
level, hyperlink, anchor).
* `\row` was ignored and only `\trowd` started a new row. Real-world
RTF often emits a single `\trowd` and separates rows with `\row`, so
every cell ended up in one row. Both `\trowd` and `\row` now begin a
fresh row (only when the current one has cells), and empty trailing
rows are dropped when closing the table.
Co-Authored-By: Claude <noreply@anthropic.com>