Adds a `horizontal` fence style that builds composite-cladding panels —
horizontal boards stacked between square intermediate posts — instead of
the vertical pickets that slat/rail/privacy all produce today.
- New `style: 'horizontal'` (core schema + inspector Style control).
- Posts march along the whole span at `postSpacing`, each with a cap.
- New `postCap` control (none/flat/pyramid, default pyramid); pyramid
caps render as a 4-sided cone fence part.
- New `slatGap` control for the board reveal — 0 collapses to one flush
panel so the stacked-board seams don't read as lines.
- `postCap` + `slatGap` are horizontal-only (visibleIf); postCap uses a
labelled dropdown so its options aren't a context-free switch.
- Lower the `postSpacing` inspector minimum from 0.2 to 0.05 m.
- 2D floor plan reuses the solid-panel marker for horizontal.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
