Prototype a modern flat client UI and identify customization gaps

Build an isolated-worktree, playable modern flat UI experiment inspired by compact WoW addon layouts. Reuse retained XML layouts, existing presenters, input and window lifecycle; preserve gameplay/network behavior. Primary outcome is evidence of UI customization capabilities and concrete limitations, not a wholesale UI rewrite. Acceptance: cohesive flat HUD/window treatment, existing actions remain usable, relevant regression/render validation, and explicit supported/blocked design goals with code evidence. Worktree: /home/mathias/Projects/worktrees/engines-client-flat-ui; branch feature/client-flat-ui-experiment. Existing checkout changes remain untouched. Implemented inherited flat control skin/tokens, labeled control-center launchers, semantic bar colors, flat hotbar decoration and glyph controls. Evidence/gaps: PRK.Client/UI_XML.md, Flat UI experiment section. Headless sample-data previews inspected at 800×600, 1440×900 and 1920×1080/150%; stable copies in .dev-data/flat-ui-preview. Validation: warnings-as-errors build; targeted client UI 177 passed/7 skipped; graphics 706 passed/1 skipped. Full client suite: 3971 passed/161 skipped/1 failure; unchanged base control reproduces identical session-boundary failure (3968 passed/161 skipped/1 failure), left unchanged. Initial character-creation failures resolved by mounting complete installation rather than GUI directory alone. New asset-free bar/icon-selection cases observed red before implementation; launcher text-fit regression observed red then corrected. No live gameplay, scene contrast, performance or cross-platform font verification. Unified horizontal unit frames, arbitrary hotbar grids, relative anchors, theme packages/reload, external icon packs and radial cooldown skinning remain outside this prototype; code evidence and constraints recorded in UI_XML.md. No commit made.