fix(dashboard): conditionally restore __Host- cookie prefix (SEC-03)

Restore the __Host- browser guarantees for the default secure deployment
without breaking plaintext/dev:
- DashboardServiceCollectionExtensions PostConfigure now resolves the cookie
  name as: explicit MxGateway:Dashboard:CookieName override wins; else
  __Host-MxGatewayDashboard when SecurePolicy==Always (RequireHttpsCookie
  true); else the plain MxGatewayDashboard default. Guard: never apply the
  __Host- prefix without Secure (browsers silently drop it).
- DashboardAuthenticationDefaults: add SecureCookieName const; keep the plain
  CookieName as the non-secure fallback.
- Docs corrected to the actual conditional contract (five stale claims):
  gateway.md, GatewayProcessDesign.md, ImplementationPlanGateway.md,
  GatewayDashboardDesign.md, CLAUDE.md; GatewayConfiguration.md phrasing
  tightened.
- Test: DashboardCookieOptionsTests asserts the name flips with
  RequireHttpsCookie and that an explicit override wins.

Server build clean (0 warnings); Dashboard tests 149/149.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Dashboard/DashboardCookieOptionsTests.cs

@@ -10,7 +10,12 @@ namespace ZB.MOM.WW.MxGateway.Tests.Gateway.Dashboard;
 
 public sealed class DashboardCookieOptionsTests
 {
-    /// <summary>Verifies that the application configures secure dashboard authentication cookies.</summary>
+    /// <summary>
+    /// Verifies that the application configures secure dashboard authentication cookies.
+    /// With <c>RequireHttpsCookie</c> defaulting to <see langword="true"/> and no explicit
+    /// <c>CookieName</c> override, the cookie is named with the <c>__Host-</c> prefix so
+    /// browsers enforce the Secure/no-Domain/Path=/ guarantees the prefix promises.
+    /// </summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
     public async Task Build_ConfiguresSecureDashboardCookie()
@@ -22,7 +27,7 @@ public sealed class DashboardCookieOptionsTests
         CookieAuthenticationOptions options = optionsMonitor.Get(
             DashboardAuthenticationDefaults.AuthenticationScheme);
 
-        Assert.Equal(DashboardAuthenticationDefaults.CookieName, options.Cookie.Name);
+        Assert.Equal(DashboardAuthenticationDefaults.SecureCookieName, options.Cookie.Name);
         Assert.True(options.Cookie.HttpOnly);
         Assert.Equal(CookieSecurePolicy.Always, options.Cookie.SecurePolicy);
         Assert.Equal(SameSiteMode.Strict, options.Cookie.SameSite);
@@ -35,11 +40,14 @@ public sealed class DashboardCookieOptionsTests
     /// <summary>
     /// Verifies that setting <c>MxGateway:Dashboard:RequireHttpsCookie=false</c>
     /// relaxes the cookie to <see cref="CookieSecurePolicy.SameAsRequest"/> so
-    /// the dashboard can be reached over plain HTTP in dev.
+    /// the dashboard can be reached over plain HTTP in dev, and that the plain
+    /// <see cref="DashboardAuthenticationDefaults.CookieName"/> is used rather than the
+    /// <c>__Host-</c> name — a <c>__Host-</c> cookie without a guaranteed Secure flag is
+    /// silently dropped by browsers.
     /// </summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
-    public async Task Build_WithRequireHttpsCookieFalse_UsesSameAsRequest()
+    public async Task Build_WithRequireHttpsCookieFalse_UsesSameAsRequestAndPlainName()
     {
         await using WebApplication app = GatewayApplication.Build(
             ["--MxGateway:Dashboard:RequireHttpsCookie=false"]);
@@ -50,12 +58,14 @@ public sealed class DashboardCookieOptionsTests
             DashboardAuthenticationDefaults.AuthenticationScheme);
 
         Assert.Equal(CookieSecurePolicy.SameAsRequest, options.Cookie.SecurePolicy);
+        Assert.Equal(DashboardAuthenticationDefaults.CookieName, options.Cookie.Name);
     }
 
     /// <summary>
-    /// Verifies that <c>MxGateway:Dashboard:CookieName</c> overrides the dashboard auth
-    /// cookie name, so a gateway instance sharing a hostname with another can be given a
-    /// distinct name (browser cookies are scoped by host+path, not port).
+    /// Verifies that an explicit <c>MxGateway:Dashboard:CookieName</c> override wins over the
+    /// secure <c>__Host-</c> default (this build leaves <c>RequireHttpsCookie</c> at its
+    /// <see langword="true"/> default), so a gateway instance sharing a hostname with another
+    /// can be given a distinct name (browser cookies are scoped by host+path, not port).
     /// </summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]