fix(style): drive the hover sheen from held state, drop the leaking start map

The sheen kept its own Dictionary<string, DateTime> of start timestamps and
only cleared an entry in the un-hover branch. A row that disappeared while the
pointer was on it left its entry behind until the plugin reloaded, which is
exactly what happens to temp tabs under the LRU limit.

It now takes the held intensity from HoverState and draws on the rising edge
only. The alpha falls off as the value climbs, so the sweep has faded out by
the time the surface underneath is fully in. On the way out it simply does not
run, which is what stops it from travelling backwards -- the old
SheenStarts.Remove prevented that by resetting, and dropping the map without
this guard would have reintroduced it.

The sidebar call site built "sidebar.tab.{guid}" per row per frame, two
allocations each. Master spec 5.3 asks for constant keys, and 7.5 for a stable
allocation count. It now uses ImGui.GetID("row"u8), which is allocation-free
and seeded from the window's ID stack, so the same literal stays distinct per
window and per PushID'd tab.

HoverSheenAllocStep pinned the old dictionary contract and is replaced by
HoverStateFootprintStep, which pins the same property against the registry:
repeated queries add no entries, and an element that stops being queried leaves
the map instead of leaking.

HellionChat/Plugin.cs

@@ -429,7 +429,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                 new SelfTests.TypingIpcStateStep(this),
                 new SelfTests.ConfigMigrationV23Step(this),
                 new SelfTests.ChannelPopoutBindStep(this),
-                new SelfTests.HoverSheenAllocStep(this),
+                new SelfTests.HoverStateFootprintStep(),
                 new SelfTests.HonorificHeaderRenderStep(this),
                 new SelfTests.AboutIntegrationsStatusStep(this),
                 new SelfTests.PerformanceBaselineStep(this),