Bento sizing model, size-derived media placement, and link tiles

Acceptance Criteria

• columns defaults to 6 for both authoring modes (author-overridable); the mode-dependent tiered=4 / span=6 split and the sizing="span" parent mode are removed.
• size presets resolve as proportions of the column count (small ⅓, medium ½, large ⅔ × 2 rows, full = all → 2/3/4/6 @ 6 cols), so they hold their ratio at any columns.
• cols / rows map to grid-column: span / grid-row: span and override size per-axis when present.
• Uniform row tracks. The grid sets grid-auto-rows: var(--rf-bento-row-height) (a fixed, themeable height — not tied to column width, which would explode vertically on collapse). Row spans (rows / large) are therefore meaningful, and a tall guest is bounded by the track (it scales/clips via WORK-339) rather than ballooning the whole row. Row height may vary per breakpoint but is never coupled to the column width.
• Author-controlled collapse mirroring split: {% bento collapse="sm|md|lg|never" %} sets the breakpoint at which the grid drops to a single stacked column (with a sensible default), replacing the hard-coded !important final collapse.
• Automatic progressive reduction. Between full grid and stacked, the column count steps down responsively and every span auto-caps via min(span, current-columns) — no per-size !important overrides — so wide cells degrade gracefully at each step. (Per-cell responsive spans are deferred — WORK-354.)
• Tiered sugar maps auto-detected relative heading depth → size preset: base → large (4×2), base+1 → medium (3×1), base+2+ → small (2×1). Deep headings do not get distinct row spans.
• media-position is author-controllable per cell (top | bottom | start | end) with a size-derived default (small → media top/stacked; large/full → prominent / beside).
• An optional href makes a whole cell a link (mirrors card), with correct focus/hover affordances and accessible markup.
• CSS keys placement off data-media-position + the resolved spans, reusing the shared split.css / card media rules where possible.
• Tests / examples cover: the 6-col default, each size preset proportion, cols/rows overrides, the tiered depth→size mapping, each media-position value, and a link tile. Existing heading-sugar bentos keep their proportions (verify).