docs: CHANGELOG + README cover the in-flight WinUI 3 redesign

CHANGELOG.md gains an Added section at the top of [Unreleased] that
walks the redesign decisions: PRODUCT/DESIGN docs, WinUI 3 scaffold,
MainWindow IA, ThemeManager, Settings drawer, Help/About/Onboarding,
HTML preview, migration plan. Calls out the WPF host as the still-
shipping build until WinUI 3 reaches feature parity.

README.md picks up:
- A Status section paragraph naming the in-flight redesign and the
  current activation blocker, with a pointer to Phase 3 of the
  migration plan
- A Build section that names both hosts so a fresh checkout doesn't
  surprise contributors with the new csproj
- Documentation section now links PRODUCT.md, DESIGN.md, the migration
  plan, and the interactive HTML preview

Both docs land BEFORE Phase 4 (view-model wiring) so onlookers
understand what's already done and what's queued.

CHANGELOG.md

@@ -6,6 +6,56 @@ adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added — Ground-up GUI redesign (started 2026-05-12)
+
+After greenlighting a from-the-scratch redesign and an explicit WinUI 3
+replatform target, the May 2026 batch is followed by a major
+restructuring of the host UI. Highlights:
+
+- **PRODUCT.md** + **DESIGN.md**: impeccable context for the redesign.
+  Solo broadcast operator at 1:50am is the canonical persona; "vibe-coded
+  GUI" is the explicit anti-reference. Tokens cover dark + light palettes
+  with context-aware accent split (cyan surface fill stays bright in
+  both modes; cyan-as-text darkens to #0E7C82 on light for AA contrast).
+- **WinUI 3 host scaffold** as `src/TeamsISO.App.WinUI/` coexisting with
+  the existing WPF host. WindowsAppSDK 1.6 LTS, unpackaged mode, win-x64
+  pinned RID, custom Bootstrap-aware `Program.Main`, post-build runtimeconfig
+  patch to drop the .NET-SDK-implicit `Microsoft.WindowsDesktop.App`
+  framework reference (WinUI 3 doesn't use it).
+- **Redesigned MainWindow**: 64px rail with brand mark + nav +
+  engine-status puck; 44px custom title bar absorbing live pills
+  (session timer · REC count · disk free) + theme toggle; section header
+  with single Primary CTA + Secondary actions; participants list
+  (ItemsRepeater stub pending DataGrid migration) at 64px row height
+  with cyan-left-border active-speaker treatment; conditional slim
+  in-call control bar; 32px status bar.
+- **ThemeManager service** holds the user theme preference
+  (System / Dark / Light), resolves via UISettings.GetColorValue when
+  System, broadcasts changes so the AppWindow title-bar buttons stay
+  in sync with the visual tree.
+- **Settings drawer** that slides in from the right (220ms ease-out-quart)
+  with five tabs (Appearance / Routing / Display / Control / Advanced).
+  Appearance tab includes the theme tri-state picker + an accent palette
+  peek. Replaces the WPF host's 380px permanent settings panel.
+- **Help / About / Onboarding** as ContentDialog-based surfaces. Help
+  is the keyboard shortcut cheat sheet; About has Wild Dragon mark +
+  version + quick-access folder shortcuts; Onboarding is three numbered
+  steps for first launch (Install NDI Runtime / Enable Teams NDI / Pick
+  transcoder topology) with "Don't show again" defaulted on.
+- **Interactive HTML preview** at `docs/preview/redesigned-mainwindow.html` —
+  faithful single-file render of the WinUI 3 XAML with theme toggle and
+  drawer interaction, so stakeholders can see the redesign before the
+  WinUI 3 build is feature-complete.
+- **Migration plan** at `docs/superpowers/plans/2026-05-12-winui3-migration.md`
+  with nine phases (scaffold / shell / activation / VM wiring / DataGrid
+  / secondary windows / hardening / tests / retire-WPF) and a risk
+  register flagging fallback paths.
+
+The WPF host (`src/TeamsISO.App/`) is unchanged and remains the shipping
+build until the WinUI 3 build passes a real-meeting smoke test. The
+view-model layer is unchanged so the WinUI 3 host will reuse it via
+ProjectReference once view-model wiring lands (Phase 4 of the plan).
+
 ### Added — May 2026 feature batch
 
 #### Engine

README.md

@@ -44,10 +44,22 @@ Pre-1.0. The May 2026 batch is feature-complete; v1.0 cut is gated on
 code-signing the MSI and a smoke pass against a real Teams meeting.
 See `CHANGELOG.md` for the [Unreleased] entry.
 
+A ground-up GUI redesign is in flight on `main` (see
+`docs/superpowers/plans/2026-05-12-winui3-migration.md`). The WPF host
+(`src/TeamsISO.App/`) remains the shipping build; a parallel WinUI 3 host
+(`src/TeamsISO.App.WinUI/`) is scaffolded with the redesigned MainWindow,
+theme system (dark + light), and secondary surfaces. Activation of the
+unpackaged WinUI 3 .exe is the current blocker — diagnostics in the
+migration plan's Phase 3.
+
 ## Build
 
-Requires .NET 8 SDK on Windows (the `TeamsISO.App` host is `net8.0-windows`
-WPF).
+Requires .NET 8 SDK on Windows. The repo has two hosts:
+
+- `src/TeamsISO.App` — WPF, `net8.0-windows`, current shipping build
+- `src/TeamsISO.App.WinUI` — WinUI 3, `net8.0-windows10.0.19041.0`, in-flight
+
+Both build together from the solution filter:
 
     dotnet restore TeamsISO.Windows.slnf
     dotnet build TeamsISO.Windows.slnf -c Release
@@ -67,6 +79,13 @@ The shipped helper scripts in the repo root automate this:
   — design overview.
 - [Embedded Teams orchestration spec](docs/superpowers/specs/2026-05-08-embedded-teams-orchestration.md)
   — Phase E roadmap.
+- [Redesign brief](PRODUCT.md) + [design system](DESIGN.md) — token-level
+  spec for the in-flight WinUI 3 redesign.
+- [WinUI 3 migration plan](docs/superpowers/plans/2026-05-12-winui3-migration.md)
+  — nine-phase plan covering scaffold through retiring the WPF host.
+- [Interactive redesign preview](docs/preview/redesigned-mainwindow.html) —
+  open in any browser to see and toggle the redesigned MainWindow before
+  the WinUI 3 binary lands.
 
 ## Keyboard shortcuts