Release v2.6.0

Added

• arrangement flow (default) - a layout's template block can now nest slots under row/column/grid containers with sizing attributes (width, height, grow, span, gap), and declare when width <compact|regular>[, height <compact|regular>] (or when height ... alone) overrides that replace the tree for a given size class. A bare template of plain slot names still parses exactly as before - this is a purely additive language change.
• arrangement freeform - a layout can instead declare variant width <compact|regular>, height <compact|regular> blocks, each placing slots at pixel-precise coordinates with place <Slot> at x,y size w,h (either dimension may be fill) or hiding one with place <Slot> hidden. One screen/data/action/form contract still applies across every variant - only placement varies.
• Width/height size classes (compact/regular) are a new two-dimensional vocabulary scoped to layout arrangement, reused across both modes so phone portrait/landscape, tablet, and desktop narrow/wide fall out of the matrix without an orientation primitive.
• A freeform layout's variant that omits a slot another variant of the same layout places (or hides) now gets a compile-time warning.
• New diagnostics PLAY0233-PLAY0244 for the above, documented in the diagnostics catalogue.
• New Layout arrangement documentation page, cross-linked from Screens.

Notes

• Scope decision: arrangement is declared per-layout, not per-slot. Mixing responsive flow for most of a layout with one pixel-precise freeform region (e.g. a chart in an otherwise-responsive dashboard) is real and common but needs slot-level arrangement - a larger grammar change tracked as a follow-up rather than folded into this PR. Documented as an explicit "out of scope for now" in the new page.
• Whether a ui profile targeting a size class with no matching variant/when should warn is a build-time concern (it depends on which layouts a profile's screens actually resolve to) - left to Stage's build pipeline, not the Screenplay compiler.
• The CI build (Release) check is expected to fail with CS9057 - a pre-existing, unrelated issue (Microsoft.Net.Compilers.Toolset pinned to 5.6.0, the GH Actions runner's SDK needs 5.9.0+). Confirmed dotnet build/dotnet test pass locally in both Debug and Release.