fix(theme): 0.3.1 — interactive-render nav backstop (issue #6)

Under an interactive Blazor render mode the runtime replaces the prerendered
<details> after DOMContentLoaded, so nav-state.js (wired on load, re-run only on
'enhancedload') never wires the live rail — no aria sync, no persistence, no
active-reveal — and native <details> content-hiding is unreliable, leaving a
collapsed section's items visible. 0.3.1:
- nav-state.js: add a MutationObserver backstop that re-runs apply() when
  details.rail-section nodes are (re)inserted; idempotent via the per-element
  init guard, loop-safe (childList-only + active-reveal's !open guard).
- layout.css: explicit .rail-section:not([open]) > .rail-section-body{display:none}
  so visual collapse works across all render modes.
- themeissues.md: document issue #6; Directory.Build.props 0.3.0 -> 0.3.1.
48 bUnit tests green.

ZB.MOM.WW.Theme/Directory.Build.props

@@ -4,7 +4,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
-    <Version>0.3.0</Version>
+    <Version>0.3.1</Version>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
   </PropertyGroup>
 </Project>