chore: baseline formatting and project settings

Phase 2 of refactor-plan.md.

- Apply solution-wide `dotnet format` (imports, whitespace).
- Add PublicApiAnalyzers package to Directory.Packages.props.
- Add src/Directory.Build.props that wires the analyzer + AdditionalFiles
  (PublicAPI.Shipped.txt / PublicAPI.Unshipped.txt) into the 8 public
  src/Arcp.* projects.
- Seed empty Shipped/Unshipped txt files; suppress RS0016/17/26/27/36/37/41
  in the root NoWarn list (will be lifted in Phase 5 when API surface is
  populated).

Test/sample csproj fixes (GenerateDocumentationFile=false, explicit
TargetFramework) were already inherited via tests/ and samples/
Directory.Build.props — nothing to add.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nficano

Directory.Build.props

@@ -10,7 +10,7 @@
     <AnalysisMode>Default</AnalysisMode>
     <DeterministicSourcePaths>true</DeterministicSourcePaths>
     <ContinuousIntegrationBuild Condition="'$(CI)' == 'true'">true</ContinuousIntegrationBuild>
-    <NoWarn>$(NoWarn);CA1014;CA1024;CA1031;CA1032;CA1034;CA1051;CA1062;CA1063;CA1064;CA1303;CA1305;CA1307;CA1308;CA1310;CA1311;CA1508;CA1707;CA1715;CA1716;CA1720;CA1721;CA1812;CA1815;CA1816;CA1822;CA1848;CA1849;CA1851;CA1852;CA1859;CA1860;CA1865;CA1866;CA1869;CA1872;CA1873;CA2007;CA2008;CA2227;CA2234;CA2237;CS1591;CS8618;CS8625;CS8629;CS8765;SYSLIB0050;NU1701;SA0001;SA1000;SA1101;SA1118;SA1124;SA1128;SA1129;SA1200;SA1201;SA1202;SA1203;SA1208;SA1210;SA1214;SA1300;SA1303;SA1309;SA1311;SA1313;SA1316;SA1401;SA1402;SA1413;SA1500;SA1501;SA1502;SA1503;SA1504;SA1505;SA1507;SA1508;SA1512;SA1513;SA1514;SA1515;SA1516;SA1517;SA1518;SA1600;SA1601;SA1602;SA1604;SA1611;SA1612;SA1614;SA1615;SA1616;SA1618;SA1619;SA1623;SA1625;SA1626;SA1629;SA1633;SA1642;SA1649;SA1652</NoWarn>
+    <NoWarn>$(NoWarn);CA1014;CA1024;CA1031;CA1032;CA1034;CA1051;CA1062;CA1063;CA1064;CA1303;CA1305;CA1307;CA1308;CA1310;CA1311;CA1508;CA1707;CA1715;CA1716;CA1720;CA1721;CA1812;CA1815;CA1816;CA1822;CA1848;CA1849;CA1851;CA1852;CA1859;CA1860;CA1865;CA1866;CA1869;CA1872;CA1873;CA2007;CA2008;CA2227;CA2234;CA2237;CS1591;CS8618;CS8625;CS8629;CS8765;SYSLIB0050;NU1701;RS0016;RS0017;RS0026;RS0027;RS0036;RS0037;RS0041;SA0001;SA1000;SA1101;SA1118;SA1124;SA1128;SA1129;SA1200;SA1201;SA1202;SA1203;SA1208;SA1210;SA1214;SA1300;SA1303;SA1309;SA1311;SA1313;SA1316;SA1401;SA1402;SA1413;SA1500;SA1501;SA1502;SA1503;SA1504;SA1505;SA1507;SA1508;SA1512;SA1513;SA1514;SA1515;SA1516;SA1517;SA1518;SA1600;SA1601;SA1602;SA1604;SA1611;SA1612;SA1614;SA1615;SA1616;SA1618;SA1619;SA1623;SA1625;SA1626;SA1629;SA1633;SA1642;SA1649;SA1652</NoWarn>
   </PropertyGroup>
 
   <PropertyGroup Label="Documentation defaults (overridden by tests)">

Directory.Packages.props

@@ -18,6 +18,10 @@
     <PackageVersion Include="System.CommandLine" Version="2.0.0-beta4.22272.1" />
   </ItemGroup>
 
+  <ItemGroup Label="Analyzers">
+    <PackageVersion Include="Microsoft.CodeAnalysis.PublicApiAnalyzers" Version="3.11.0-beta1.23420.2" />
+  </ItemGroup>
+
   <ItemGroup Label="Test">
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
     <PackageVersion Include="xunit" Version="2.9.2" />

src/ARCP.Cli/PublicAPI.Shipped.txt

@@ -10,12 +10,12 @@
 using System.Threading.Tasks;
 using Arcp.Core.Auth;
 using Arcp.Core.Caps;
-using Arcp.Core.Wire;
 using Arcp.Core.Errors;
 using Arcp.Core.Ids;
 using Arcp.Core.Leases;
 using Arcp.Core.Messages;
 using Arcp.Core.Transport;
+using Arcp.Core.Wire;
 
 namespace Arcp.Client;
 

src/ARCP.Cli/PublicAPI.Unshipped.txt

@@ -1,8 +1,8 @@
 // SPDX-License-Identifier: Apache-2.0
 using System;
 using System.Text.Json;
-using Arcp.Core.Wire;
 using Arcp.Core.Messages;
+using Arcp.Core.Wire;
 
 namespace Arcp.Client;