Fix: table cells serialize empty in JSON output

[sh3raawii]

Summary

Border-detected table cells serialize with empty kids in JSON output, dropping
all cell text — even though the same tables render correctly in Markdown, HTML,
and text.

Root cause

table_cell_to_legacy builds a cell's kids from cell.contents
(Vec<ContentElement>), and elements_to_legacy intentionally skips raw
TextChunk / TextLine / TextBlock elements. But the border-detection
content-assignment path (table_content_assigner::assign_to_cell) stores a
cell's text in the other field, cell.content (Vec<TableToken>):

ContentElement::TextChunk(tc) => {
    cell.content.push(TableToken { base: tc.clone(), token_type: TableTokenType::Text });
}

So for any table populated by the border detector, cell.contents is empty and
the token text in cell.content is never emitted → every cell comes out as
"kids": []. Markdown/HTML/text read cell.content, so they are unaffected;
only JSON loses the data.

Fix

In table_cell_to_legacy, when contents yields no kids, fall back to the
cell's content tokens and synthesize a paragraph kid from their text. This
mirrors the fallback the same file already uses for list items ("First tries
semantic contents (preferred), then falls back to raw body tokens").

Before / after

A born-digital 6×2 key-value table (Input | Value, Equity volatility | 55%,
…) that Markdown renders in full previously produced 12 empty cells in JSON.
After the fix, each cell's kids contains a paragraph node with the cell text.

Changes

• crates/edgeparse-core/src/output/legacy_json.rs — table_cell_tokens_to_paragraph
  helper and a fallback in table_cell_to_legacy.