layout: implement table rowspan geometry (#357)

[carlos7ags]

Fixes #357.

Problem

rowspan was stored and used only to reserve grid occupancy so following rows skipped the spanned columns. The vertical geometry was never implemented: a spanning cell contributed height to its starting row only and was always drawn one row tall, so it appeared as a single row.

Fix

• Add spanHeight to gridCell (the vertical analogue of spanWidth).
• In buildGrid, exclude rowspanning cells from their starting row's natural height, then resolve span heights in a second pass: sum the spanned rows' heights plus the inter-row spacing gaps, growing the last spanned row when the cell's content needs more room.
• drawTableRowDirect draws spanning cells at spanHeight. Vertical alignment already keys off the height argument, so content centers across the full span.

Preview

The new examples/table-rowspan demo (issue's minimal case + a 3-row schedule grid):

Tests

• TestTableRowspan — span covers both rows; non-spanning sibling has spanHeight == 0.
• TestTableRowspanIncludesVerticalSpacing — span includes the inter-row gap.
• TestTableRowspanDeficitGrowsLastRow — tall content grows the last spanned row.
• TestTableRowspanThreeRows — 3-row span covers all rows plus both gaps.
• TestTableColspanAndRowspanCombined — width and height span independently.
• Example smoke test plus existing layout/colspan suites pass.

Known limitation

A rowspan that straddles a page break is not yet handled — PlanLayout's split logic is span-unaware, so such a cell draws its full height past the page bottom. Documented in a code comment; tracked in #362.