fix(docs): markdown empty-header tables leak last row after the table

[chrischall]

Closes #609.

Root cause

isTableSeparator accepted rows of empty pipe cells like | | | as separators because the per-segment loop hit continue for empty segments and never tripped the "must have at least one dash" check at the end of the loop body. Concretely:

for _, seg := range segments {
    seg = strings.TrimSpace(seg)
    if seg == "" {
        continue          // ← skips the dash check
    }
    // … dash-only check + must-have-a-dash check …
}
return len(segments) > 1

With input

|     |     |       ← row 0 (empty header) — also matched isTableSeparator
|-----|-----|       ← row 1 (real separator)
| Label A | Value A |
| Label B | Value B |

parseMarkdownTable then skipped both row 0 (false separator) and row 1, returning only 2 data rows. The outer parser advanced i += len(tableCells) = 2, putting the next iteration on row 3, where | Label B | Value B | was re-parsed as a literal pipe paragraph.

Fix

Track sawDashSegment in isTableSeparator and require at least one non-empty, dash-containing segment. || is still rejected by the existing len(segments) > 1 guard, so this change is strictly additive.

Tests

• TestIsTableSeparator_EmptyPipeRowRejected — table-driven cases covering empty-cell forms, normal separators, and alignment variants.
• TestParseMarkdown_EmptyHeaderTableKeepsAllDataRows — full ParseMarkdown regression that exercises the exact repro from the issue and asserts there is exactly one MDTable element with all three rows intact.

go test ./internal/cmd/ -count=1 → ok 23.5s.