Bump CosmoKv ref to 2.1.9 (deep-dive Phase 1-3 fixes)

CosmoKv 2.1.6→2.1.9 brings:
  - Phase 1: writer-channel orphan-TCS drain, compactor degraded-state
    backpressure-loop liveness check, DisposeAsync ordering fix.
  - Phase 2: pooled List<TxnEntry> on writer task hot path, new
    LevelsController.AllTables(List<T>) overload avoiding ToArray per
    IterateAsync call.
  - Phase 3: Compactor lock-scope refactor — _compactionLock now held
    only on plan + swap, build phase runs lock-free. NumCompactors>1
    actually parallelises on disjoint levels.

All 266 SQL/CosmoKv tests pass against the new driver.

Author: vkuttyp

Directory.Build.props

@@ -7,6 +7,6 @@
     <Authors>vkuttyp</Authors>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <RepositoryUrl>https://github.com/vkuttyp/CosmoSQLClient-Dotnet</RepositoryUrl>
-    <Version>2.5.4</Version>
+    <Version>2.5.5</Version>
   </PropertyGroup>
 </Project>

src/CosmoSQLClient.CosmoKv/CosmoSQLClient.CosmoKv.csproj

@@ -5,7 +5,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="CosmoKv" Version="2.1.5" />
+    <PackageReference Include="CosmoKv" Version="2.1.9" />
   </ItemGroup>
 
   <PropertyGroup>