feat(ui): AcDream.UI.Abstractions layer — IPanel / IPanelRenderer / VitalsVM

Adds the backend-agnostic UI contract layer called for by the 2026-04-24
staged UI strategy (docs/plans/2026-04-24-ui-framework.md). This is the
stable layer both the Phase D.2a Hexa.NET.ImGui backend and the later
D.2b custom retail-look backend implement.

New module `src/AcDream.UI.Abstractions/`:

  * IPanel        — a drawable panel (id/title/visible/Render)
  * IPanelHost    — owns the panel list, drives per-frame dispatch
  * IPanelRenderer — drawing primitives (Begin/End/Text/SameLine/
                    Separator/ProgressBar). Kept small + retail-friendly
                    on purpose — if a widget can't be expressed with
                    dat-sourced sprites+fonts later, don't add it here.
  * ICommandBus   — user-intent publisher; NullCommandBus is D.2a default
  * PanelContext  — per-frame record struct (DeltaSeconds + Commands)
  * Panels/Vitals/
      VitalsVM   — reads CombatState.GetHealthPercent for the local
                   player. Stamina/Mana return null in D.2a; they await
                   a LocalPlayerState cache of PlayerDescription (0x0013)
                   which is filed as a follow-up issue.
      VitalsPanel — first real panel. HP bar always drawn; Stam/Mana
                    appear automatically when the VM returns non-null.

Invariant documented in IPanel's XML doc: no `using Hexa.NET.ImGui` in
panel files, ever. If a widget needs something IPanelRenderer can't
express, the interface grows; panels never reach through.

References AcDream.Core for CombatState. Zero runtime/UI dependencies
— this project compiles headless, perfect for unit testing the
ViewModels.

No visible change yet. Next commits: (2) tests, (3) ImGui backend,
(4) GameWindow hookup + visible panel behind ACDREAM_DEVTOOLS=1.

src/AcDream.UI.Abstractions/Panels/Vitals/VitalsPanel.cs

@@ -0,0 +1,69 @@
+namespace AcDream.UI.Abstractions.Panels.Vitals;
+
+/// <summary>
+/// First real UI panel — shows the local player's vitals as progress bars.
+/// Backend-agnostic; renders exclusively through <see cref="IPanelRenderer"/>
+/// so the same file works under Hexa.NET.ImGui (D.2a) and the future custom
+/// retail-look toolkit (D.2b).
+///
+/// <para>
+/// D.2a shows only HP (percent). <see cref="VitalsVM.StaminaPercent"/> /
+/// <see cref="VitalsVM.ManaPercent"/> return null until a
+/// <c>LocalPlayerState</c> is wired (follow-up issue). When they start
+/// returning non-null, this panel picks them up automatically.
+/// </para>
+/// </summary>
+public sealed class VitalsPanel : IPanel
+{
+    private const float BarWidth = 200f;
+
+    private readonly VitalsVM _vm;
+
+    public VitalsPanel(VitalsVM vm)
+    {
+        _vm = vm ?? throw new ArgumentNullException(nameof(vm));
+    }
+
+    /// <inheritdoc />
+    public string Id => "acdream.vitals";
+
+    /// <inheritdoc />
+    public string Title => "Vitals";
+
+    /// <inheritdoc />
+    public bool IsVisible { get; set; } = true;
+
+    /// <inheritdoc />
+    public void Render(PanelContext ctx, IPanelRenderer renderer)
+    {
+        if (!renderer.Begin(Title))
+        {
+            renderer.End();
+            return;
+        }
+
+        // HP — always available from CombatState.
+        float hp = _vm.HealthPercent;
+        renderer.Text("HP");
+        renderer.SameLine();
+        renderer.ProgressBar(hp, BarWidth, overlay: $"{hp * 100f:F0}%");
+
+        // Stamina — show only when the VM has a real value.
+        if (_vm.StaminaPercent is float stam)
+        {
+            renderer.Text("Stam");
+            renderer.SameLine();
+            renderer.ProgressBar(stam, BarWidth, overlay: $"{stam * 100f:F0}%");
+        }
+
+        // Mana — show only when the VM has a real value.
+        if (_vm.ManaPercent is float mana)
+        {
+            renderer.Text("Mana");
+            renderer.SameLine();
+            renderer.ProgressBar(mana, BarWidth, overlay: $"{mana * 100f:F0}%");
+        }
+
+        renderer.End();
+    }
+}