feat(editor): align existing UI with Figma and harden interactions

[JeanPhilippeKernel]

Summary

• Align the existing editor chrome, typography, menus, tabs, Outliner, Details, Content Browser, Asset Importer, status bar, and Settings with the approved Figma UI.
• Complete existing editor behavior: import controls and progress, recent levels, focus-selected, panel visibility/reset, docking, file drag-and-drop, and submenu dismissal.
• Add the approved RGB/HSB/HSL color editor: live hexagon/chain interaction, hue and vertex selection, numeric/hex input, linear/sRGB storage boundaries, gesture undo/redo, old/new previews, and bounded recent colors.
• Harden rendering and input: per-list arena ownership, bounded 16-bit geometry/tessellation, explicit initialization, secure memory helpers, reduced scratch/zeroing work, retained growth capacity, and platform-aware shortcuts.
• Tighten index-buffer capacity to the proven worst-case geometry density, saving 576 KiB across three GPU upload slots, and avoid repeated picker child scans/ID hashing.
• Include font/icon/color-math license attribution and regression coverage. New editor types and gizmo work remain out of scope.

Validation

• macOS arm64 Debug: Obelisk, ZEngineTests, and TetragramaTests build successfully.
• Engine suite: 1,094 passed; 4 existing tests skipped because they require a real Vulkan device (1,098 total).
• Editor suite: all 58 tests passed, including asset importer and Settings coverage.
• UndefinedBehaviorSanitizer: all 187 targeted UI/rendering/math/Settings tests passed.
• Pre-push clang-format checks passed for ZEngine, Tetragrama, Obelisk, and shaders; Obelisk rebuilt successfully after the shader-formatting-only follow-up.
• Added regressions for arena isolation, capacity retention, stale payload clearing, 16-bit limits, dirty-arena initialization, bounded scratch/tessellation, worst-case index density, and same-frame picker model styling.
• Native Windows/Linux validation has not been run locally; CI results are pending.

Local screenshots, the UI implementation design document, and documentation navigation edits are intentionally excluded.