Adopt Stryker.NET as a repo-local tool, for the mutation-testing spike

Pinned in dotnet-tools.json (dotnet-stryker 5.0.0) so any checkout reproduces
it with `dotnet tool restore`. Reports land in StrykerOutput/, now ignored.

Spike against FireControl.cs alone: 276 mutants in 7m18s, 208 killed, 67
survived, 4 uncovered, score 74.6%. It killed every mutation the hand-written
anchor harnesses defended, compiles all mutants into one assembly and switches
between them at runtime -- so it never edits a source file, has nothing to
re-anchor after a refactor, and cannot leak a mutant into the tree.

Among the survivors: Splash's handedness is untested (flipping the sign of
`right` passes every test -- the operator's "damage on the wrong side of the
ship"), deleting the discrete shield's TakeHit survives, and most of the rest
trace to degenerate fixture values -- shooter at the origin, fire time zero,
spread zero -- which make whole families of mutants arithmetically identical.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -109,3 +109,6 @@ tests/_scratch_*
 .nuget-sandbox/
 .dotnet-sandbox/
 .appdata-sandbox/
+
+# Stryker.NET mutation reports (regenerated per run)
+StrykerOutput/

dotnet-tools.json

@@ -0,0 +1,13 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "dotnet-stryker": {
+      "version": "5.0.0",
+      "commands": [
+        "dotnet-stryker"
+      ],
+      "rollForward": false
+    }
+  }
+}