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.Server/Dashboard/DashboardAuthenticationDefaults.cs

@@ -35,5 +35,23 @@ public static class DashboardAuthenticationDefaults
 
     public const string LdapGroupClaimType = "mxgateway:ldap_group";
     public const string KeyPrefixClaimType = "mxgateway:key_prefix";
+
+    /// <summary>
+    /// Dashboard auth cookie name used when the cookie is not guaranteed to be Secure
+    /// (<c>RequireHttpsCookie=false</c> → <see cref="Microsoft.AspNetCore.Authentication.Cookies.CookieSecurePolicy.SameAsRequest"/>)
+    /// or when an explicit <c>MxGateway:Dashboard:CookieName</c> override is absent but the
+    /// secure default cannot be applied. This plain name carries no browser-enforced guarantees.
+    /// </summary>
     public const string CookieName = "MxGatewayDashboard";
+
+    /// <summary>
+    /// Dashboard auth cookie name applied when the cookie is guaranteed Secure
+    /// (<c>RequireHttpsCookie=true</c> → <see cref="Microsoft.AspNetCore.Authentication.Cookies.CookieSecurePolicy.Always"/>)
+    /// and no explicit <c>MxGateway:Dashboard:CookieName</c> override is set. The <c>__Host-</c>
+    /// prefix instructs browsers to enforce Secure, no <c>Domain</c>, and <c>Path=/</c>; those
+    /// guarantees only hold for a Secure cookie, so this name must never be applied unless
+    /// <see cref="Microsoft.AspNetCore.Authentication.Cookies.CookieSecurePolicy.Always"/> is in
+    /// effect — a <c>__Host-</c> cookie without Secure is silently dropped by browsers.
+    /// </summary>
+    public const string SecureCookieName = "__Host-MxGatewayDashboard";
 }

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardServiceCollectionExtensions.cs

@@ -123,13 +123,22 @@ public static class DashboardServiceCollectionExtensions
                     ? CookieSecurePolicy.Always
                     : CookieSecurePolicy.SameAsRequest;
 
-                // Config-driven cookie name (MxGateway:Dashboard:CookieName). Null/blank keeps
-                // the canonical default set above, so a misconfiguration cannot unname the cookie.
+                // Config-driven cookie name (MxGateway:Dashboard:CookieName). An explicit override
+                // always wins. With no override, restore the __Host- prefix when the cookie is
+                // guaranteed Secure (RequireHttpsCookie true → SecurePolicy Always): the __Host-
+                // browser guarantees (Secure required, no Domain, Path=/) hold only for a Secure
+                // cookie, and a __Host- cookie without Secure is silently dropped — so the prefix
+                // is never applied unless SecurePolicy is Always. Otherwise keep the plain
+                // canonical default set by AddCookie above, so a misconfiguration cannot unname it.
                 var cookieName = gatewayOptions.Value.Dashboard.CookieName;
                 if (!string.IsNullOrWhiteSpace(cookieName))
                 {
                     cookieOptions.Cookie.Name = cookieName;
                 }
+                else if (cookieOptions.Cookie.SecurePolicy == CookieSecurePolicy.Always)
+                {
+                    cookieOptions.Cookie.Name = DashboardAuthenticationDefaults.SecureCookieName;
+                }
             });
 
         services.AddAuthorization(authorization =>