refactor: centralize build config and pin the SDK (#156) (#182)

Adds global.json (10.0 SDK, rollForward latestFeature) so local dev
resolves the same SDK line CI pins in five workflows, instead of
floating to whatever is installed.

Adds Directory.Build.props for LangVersion/Nullable, the two
properties identical across all three csproj (TargetFramework stays
per-project since the test project multi-targets net10.0;net48).

Adds Directory.Packages.props with central package management, moving
every PackageReference version out of the three csproj into one file.
Newtonsoft.Json bumped 13.0.3 -> 13.0.4 in the single place instead of
two independent declarations that could drift. xunit.runner.visualstudio
stays at 4.0.0 (it runs xUnit v1/v2/v3 per its own description; nothing
in the issue's stated xunit 2.9.3 pairing required a downgrade).

Removes the self-referential $(NoWarn) from the Core csproj, a no-op.

Adds an explicit System.Memory 4.6.3 PackageReference to the net48
leg of the test project, which resolves the MSB3277 conflict between
System.Memory 4.0.1.2 and 4.0.5.0 (110 warnings -> 0). Scoped to net48
only per the issue, which names only the net48 test target.

Adds PackageVersionCentralizationTests, an xUnit test asserting none
of the three csproj declare a PackageReference Version attribute
outside Directory.Packages.props.

Co-authored-by: goodolclint-claude[bot] <323206664+goodolclint-claude[bot]@users.noreply.github.com>

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "10.0.100",
+    "rollForward": "latestFeature"
+  }
+}