NEO-146: fix mutation outcome docs and null-check style

Add summary and Kind param XML docs on mutation outcome records, simplify
PostgresDockerHelper null guard with ?? throw, and document RCS1139/IDE0270.

.cursor/rules/csharp-style.md

@@ -133,8 +133,9 @@ var store = secondScope.ServiceProvider.GetRequiredService<IExampleStore>(); //
 ## Documentation
 
 - Use **`///` XML doc comments** on public APIs (types and members) when behavior or contracts are not obvious.
+- **Primary-constructor records:** include a **`<summary>`** plus **`<param>`** for every constructor parameter (Roslynator RCS1139 / RCS1263). Do not leave orphaned `<param>` lines without a summary on the type.
 
-## Test project layout (`*.Tests`)
+## Null checks (IDE0270) (`*.Tests`)
 
 - **Mirror the server project:** place test types under the same relative path as the production code they exercise (e.g. `NeonSprawl.Server/Game/PositionState/PositionStateApi.cs` → `NeonSprawl.Server.Tests/Game/PositionState/PositionStateApiTests.cs`).
 - Use a namespace that matches the folder tree under the test assembly root, e.g. `NeonSprawl.Server.Tests.Game.PositionState` for files in `Game/PositionState/`.

server/NeonSprawl.Server.Tests/Game/PositionState/PostgresDockerHelper.cs

@@ -85,12 +85,9 @@ internal static class PostgresDockerHelper
             return;
         }
 
-        var root = FindDockerComposeRoot();
-        if (root is null)
-        {
-            throw new InvalidOperationException(
+        var root = FindDockerComposeRoot()
+            ?? throw new InvalidOperationException(
                 "Postgres is not reachable and docker-compose.yml was not found by walking up from the test output directory.");
-        }
 
         await RunDockerComposeAsync(root, "compose up -d", CancellationToken.None).ConfigureAwait(false);
 

server/NeonSprawl.Server/Game/Gathering/ResourceNodeInstanceMutationOutcome.cs

@@ -10,6 +10,8 @@ public enum ResourceNodeInstanceMutationKind
     Applied,
 }
 
+/// <summary>Result of <see cref="ResourceNodeInstanceOperations.TryCommitSuccessfulGatherDecrement"/> (NEO-61).</summary>
+/// <param name="Kind">Applied or denied.</param>
 /// <param name="ReasonCode">Populated when <see cref="Kind"/> is <see cref="ResourceNodeInstanceMutationKind.Denied"/>.</param>
 /// <param name="Snapshot">Authoritative snapshot after apply, or current snapshot on depletion deny; null for unknown node.</param>
 public readonly record struct ResourceNodeInstanceMutationOutcome(

server/NeonSprawl.Server/Game/Items/PlayerInventoryMutationOutcome.cs

@@ -13,6 +13,8 @@ public enum PlayerInventoryMutationKind
     Applied,
 }
 
+/// <summary>Result of <see cref="PlayerInventoryOperations.TryAddStack"/> or <see cref="PlayerInventoryOperations.TryRemoveStack"/> (NEO-54).</summary>
+/// <param name="Kind">Applied, denied, or store missing.</param>
 /// <param name="ReasonCode">Populated when <see cref="Kind"/> is <see cref="PlayerInventoryMutationKind.Denied"/>.</param>
 /// <param name="Snapshot">Authoritative snapshot after apply, or unchanged snapshot on deny; null when store missing.</param>
 public readonly record struct PlayerInventoryMutationOutcome(