release prep: 9.15.1 — fix ForTenant dropping tenancy-neutral documents (GH-4947)

Patch release for a silent data-correctness regression. A ForTenant() view of an
identity/dirty-tracked session stopped seeing tenancy-neutral (global) documents
tracked by the parent session, so LoadAsync returned null for a document that
exists. Broken since 9.13.0 (74b11a4 / #4807 tenant-scoped the identity map
per SESSION rather than per DOCUMENT TYPE). Conjoined isolation from #4801 is
preserved exactly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jeremydmiller

Directory.Build.props

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project>
     <PropertyGroup>
-        <Version>9.15.0</Version>
+        <Version>9.15.1</Version>
         <LangVersion>13.0</LangVersion>
         <Authors>Jeremy D. Miller;Babu Annamalai;Jaedyn Tonee</Authors>
         <PackageIconUrl>https://martendb.io/logo.png</PackageIconUrl>