feat(rust): honor width / height / fill on row + column scene layouts

[esengine]

Unblocks multi-pane scene layouts. Foundation PR for the
`Sidebar / Main / ContextPanel` three-pane layout the user's
design mock points at.

Why

The scene schema already declared `width` / `height` on
`BoxLayout` as `Dim::Cells(n) | Dim::Fill`, but the Rust render
layer ignored both — every row child fell back to its intrinsic
width and every column child to its intrinsic height. That made
`direction: row` only useful for content-packed rows, never for
sized panes (244-cell sidebar + fluid main + 320-cell context panel
was literally not expressible end-to-end).

Algorithm

Single-pass distribution per axis:

1. Reserve gap_total = gap × (n − 1) up front.
2. Walk children; classify each:
   • `Dim::Cells(n)` → reserve n cells, clamped at remaining
     (first-come basis, source order).
   • `Dim::Fill` → defer.
   • unsized → reserve intrinsic size, clamped at remaining.
3. Split whatever's left equally among deferred Fill children.
   Pixel remainder goes to the leading Fill children so the total
   adds up to exactly the gap-adjusted axis budget.

Backward compatible — every existing scene (column with unsized
children, row with intrinsic-width chips) keeps rendering identically.

Visible use

`titleRow` is now a row box: brand on the left, model name on the
right, `width: "fill"` spacer between them. Same pattern the
upcoming three-pane layout PR will use — sidebar with
`Dim::Cells(n)`, main with `Dim::Fill`, ctx panel with its own
fixed cell width.

Tests

`crates/reasonix-render/tests/render.rs` — 9 new cases:

• fixed cell widths reserved in source order
• one Fill child consumes all remaining row width
• multiple Fill children split remainder evenly
• uneven remainder goes to leading Fill children
• fixed width clamped at remaining when oversized
• unsized children keep their intrinsic widths
• gap subtracted from available before distributing
• column variant for both Cells + Fill

`tests/scene-trace-frame.test.ts` — 2 new cases on the producer-side
`titleRow` restructure.

`cargo test -p reasonix-render --test render` → 17/17 green.
`npm run verify` → green via prepush gate.

What still doesn't work

• `flexGrow` / `flexShrink` — schema fields exist, render layer
  still ignores them. `Dim::Fill` is the primary way to grow.
• `marginX` / `marginY` — same.
• `align` / `justify` — same. (Right-alignment in a row is
  currently done by inserting a Fill spacer before the right-aligned
  child, as the new `titleRow` does.)
• `borderStyle` / `borderColor` — same.

Each of these is a small isolated follow-up; `width` / `height`
was the highest-impact one because it unlocks fixed-sized panes.

Refs #868