Pin mtp test-runner for CultMath and document mutation testing

CultMath.Tests is on xunit.v3. Stryker's default vstest runner cannot
capture coverage against xunit.v3's test host and hangs retrying test
sessions indefinitely (observed: 2096 mutants queued, zero completed after
~9 minutes of retries). Stryker's Microsoft.Testing.Platform runner
(-t mtp, still preview) works correctly against xunit.v3 and completed the
same 1351-mutant run in ~4.5 minutes. Pin it in config so a plain
`dotnet stryker` works without a CLI flag.

Add docs/mutation-testing.md: how to run it, the --since form for a single
cut, the triage rules, a known coverage-capture caveat in the mtp runner
(confirmed by hand-mutation against static readonly fields), and the
packages still needing their own config.

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

docs/mutation-testing.md

@@ -0,0 +1,86 @@
+# Mutation Testing
+
+CultLib measures its C# test suites with Stryker.NET, pinned as a repo-local
+tool in `dotnet-tools.json`. Coverage is added project by project; a project
+with no `stryker-config.json` next to its test project has not been scoped
+yet (see "Follow-ups" below for what's outstanding).
+
+## Running it
+
+Restore the tool once per checkout:
+
+```powershell
+dotnet tool restore
+```
+
+Run a full mutation pass from a covered test project's directory, for
+example CultMath:
+
+```powershell
+cd packages/cultmath/tests/CultMath.Tests
+dotnet stryker -t mtp
+```
+
+`-t mtp` selects Stryker's Microsoft.Testing.Platform runner. It is required
+for test projects on `xunit.v3` (CultMath.Tests is one): Stryker's default
+`vstest` runner's coverage capture does not work against xunit.v3's native
+test host and the run stalls retrying test sessions indefinitely. Projects
+still on `xunit` v2 with `xunit.runner.visualstudio` can use the default
+`vstest` runner.
+
+For a single cut, scope the run to what changed instead of the whole
+project:
+
+```powershell
+dotnet stryker -t mtp --since:<base-commit-or-branch>
+```
+
+Reports land in `StrykerOutput/` next to the test project (git-ignored).
+Open the `reports/mutation-report.html` file for a browsable view, or read
+`reports/mutation-report.json` for a scriptable one.
+
+## Triage rules
+
+The mutation score is not a gate (`break` threshold is 0). Every surviving
+mutant is a finding about the tests, not a build failure, and gets triaged
+into one of:
+
+- **missing behavioural test** — name the test that should exist and what
+  branch, edge case, or component it would exercise.
+- **degenerate fixture** — the code path is exercised, but the fixture's
+  values make two different mutants produce the same observable result (a
+  zero component that erases a `+`/`-` distinction, an already-normalized
+  input that erases a `*`/`/` distinction, all-equal or all-same-sign inputs
+  that erase a boundary or `&&`/`||` distinction). Fix the fixture, don't
+  invent a new one that only kills that one mutant.
+- **equivalent** — the mutant cannot change behaviour. Float-threshold
+  boundary flips (`>` to `>=`) are equivalent by default. A reasoned
+  equivalence claim (for example, `>>` and `>>>` on an already-unsigned
+  operand) needs its one-line reason recorded, not assumed.
+
+Do not write a hand-rolled mutation suite or anchor test as a substitute.
+Where the tool can't reach (native/shader/editor code), the defence is a
+behavioural test at the layer the rule is decided, not a committed mutation
+harness.
+
+## Known tool caveat
+
+Stryker 5.0.0's `mtp` runner is documented as preview. Its per-test coverage
+capture was found to report false "Survived" verdicts specifically for
+mutations inside `static readonly` field initializers that a test does
+cover (confirmed by hand-mutating `bool3.@false` and `bool4.@true` and
+running the full suite directly with `dotnet test`, both of which fail
+against the mutation that Stryker reported as surviving). Treat a survivor
+inside a `static readonly` field initializer as unproven until confirmed
+with a direct `dotnet test` run against the same mutation; do not triage it
+as a missing test without that check.
+
+## Follow-ups
+
+CultMath.Tests is the only project currently scoped. `GameCult.Caching`,
+`GameCult.Geometry`, `GameCult.Mesh` (+ `.Quic`, `.Quic.Native`),
+`GameCult.Networking` (+ `.WebSockets`), and the non-.NET runtimes
+(`cultcache-rs/py/ts`, `cultmesh-*`, `cultnet-*`) each need their own
+`stryker-config.json` (or ecosystem-equivalent tool: `cargo-mutants` for
+Rust, `StrykerJS` for TypeScript, `mutmut` for Python) and a baseline pass.
+None of that is wired into CI yet.

packages/cultmath/tests/CultMath.Tests/stryker-config.json

@@ -8,6 +8,7 @@
     ],
     "mutation-level": "Standard",
     "reporters": [ "progress", "html", "json" ],
-    "thresholds": { "high": 80, "low": 60, "break": 0 }
+    "thresholds": { "high": 80, "low": 60, "break": 0 },
+    "test-runner": "mtp"
   }
 }