feat(data-table): lay one element across a whole row

[kirillDevPro]

What & why

Every MoonDataTable cell is overflow_hidden — that is what keeps a long value out of its
neighbour's column. It also means a row that wants to say ONE thing across its full width cannot
say it through a cell: a section heading placed in the leftmost column is cut at that column's
edge no matter how much empty room the rest of the row has. Fixing it from the caller's side is
not possible either, because the final render widths are resolved inside the table from a
measured viewport (auto_width_columns), so any offset computed outside is wrong exactly on a
narrow panel.

MoonTableRow::banner / MoonDataRow::banner take one element and paint it across the row,
after the cells and outside their clipping.

Two design points worth reviewing:

• The banner does not occlude. GPUI stops hit-testing only at a HitboxBehavior::BlockMouse
  hitbox, set solely by occlude(), and this wrapper calls neither — so a cell underneath stays
  clickable. The consequence to design around is the opposite of occlusion: a handler on the
  banner and one on a cell beneath it both fire for a single click. Documented on the builder.
• has_banner is a pub(crate) predicate, not an exposed field. The forward crosses a module
  boundary and nothing below it is observable without a rendering harness, so the conversion would
  otherwise be untestable; a public field would let any module in the crate move the element out
  of a row it does not own. It carries #[cfg_attr(not(test), allow(dead_code))] — conditional, so
  a real orphaning still warns.

How to verify

cargo test -p moon-ui-components          # 431 passed, 0 failed
cargo run -q -p xtask -- component-api --check-baseline   # PASS
powershell -ExecutionPolicy Bypass -File tools\run-component-guardrails.ps1   # PASS

data_row_conversion_preserves_banner_presence is mutation-proven: deleting the forward in
as_table_row reddens it, and it asserts both directions so a mutation that hard-codes true
cannot pass. The baseline refresh adds exactly the two new banner() signatures.