fix(SEC-41, SEC-40): re-resolve dashboard tag grants; gate the alarm feed on a real role

Committed together because the two changes share the same doc paragraphs (the ACL
section of GatewayDashboardDesign.md, the GroupToTag / AllowAnonymousLocalhost rows
of GatewayConfiguration.md, and CLAUDE.md's authentication paragraph), and splitting
them would leave one of the two describing behaviour that does not yet exist.

SEC-41 — tag grants no longer trusted from login-stamped claims
---------------------------------------------------------------
DashboardSessionAcl read the zb:dashboardtag claims stamped into the cookie at
login. GatewayOptions binds once at startup, so applying a GroupToTag edit needs a
restart — but the restart does not clear the auth cookie (data-protection keys
persist), so a browser returning afterwards re-authenticated with tag claims
resolved under the OLD map. A revoked grant therefore survived on the cookie seam
for the sliding 8-hour idle life, renewable indefinitely by activity, while the
hub-token seam had no such gap because its mint re-resolves.

CanViewSession now re-resolves the grant on every subscribe from the caller's
mxgateway:ldap_group claims through DashboardGroupTagMapping.MapGroupsToTags
against the currently bound map — the same one call HubTokenService.Issue makes.

Fallback (documented on the class and in the design doc): a principal carrying NO
group claims falls back to its stamped tag claims. That arm exists for the
hub-token principal — HubTokenService.Validate rehydrates roles and tags but not
groups, so re-resolution already happened at mint, one lifetime ago at worst. It
cannot re-open the cookie hole: DashboardAuthenticator stamps group and tag claims
from one group list, so a cookie principal holding a tag claim always holds the
groups it came from, and a login yielding no groups yields no tags. A principal
with neither kind of claim stays an empty-grant Viewer.

SEC-40 — the alarm feed requires a real authenticated dashboard role
--------------------------------------------------------------------
AlarmsHub sat behind HubClientsPolicy, which the read-only environment bypasses
satisfy, so an anonymous-localhost caller or a Mode=Disabled deployment received
galaxy-wide alarm metadata — alarm reference (a full tag path), state, severity,
category, area, acknowledging operator — for the whole subscription expression.

Decided: tighten the gate, keep the scope. New AuthenticatedDashboardRoleRequirement
+ its handler, and a MxGateway.Dashboard.Alarms policy built from it. Because
DashboardAuthorizationHandler is typed to DashboardAuthorizationRequirement, it is
never invoked for the new type, so neither Authentication:Mode=Disabled nor
anonymous localhost can satisfy the policy — the same structural device the mounted
Secrets UI already relies on. The policy keeps both schemes so external hub clients
can still authenticate with a bearer token, and DisableLogin auto-login is
unaffected (it mints a genuinely authenticated both-roles principal).

The /alarms page carries the same policy, not just the hub: it reads the same
central monitor in process, so gating only the hub would have left the disclosure
one navigation away. The side rail hides the Alarms link for callers the policy
denies, matching the Secrets link precedent. AlarmsHubPublisher is unchanged and
stays unfiltered by design — audience is decided at hub admission, and the monitor
carries no per-session provenance to narrow by.

Alarm metadata deliberately remains galaxy-wide for authenticated Viewers: an
AlarmFeedMessage has no session id or owning key for the SEC-25 tag intersection to
work against, and equating a visibility tag with an alarm-reference prefix would
turn GroupToTag into the data-access constraint it explicitly is not.

Tests
-----
DashboardSessionAclTests: the SEC-41 regression — a cookie-style principal whose
stale tag claim is no longer granted by any of its groups under the current map is
denied; plus the allowing direction (including with no stamped tag at all, which
proves the decision comes from the groups), the ignore-case corner, group claims
that map to nothing, and the hub-token fallback asserted against an EMPTY map so
dropping the fallback would fail loudly.

AlarmFeedAuthorizationTests (new): both handlers run together in the most permissive
environment (Mode=Disabled + loopback + AllowAnonymousLocalhost) do not satisfy the
alarm requirement, with the bypassable requirement as the control so the negative
cannot pass because the bypass was misconfigured; authenticated Viewer/Admin do
satisfy it; an unauthenticated role claim does not; the registered policy carries
the non-bypassable requirement and both schemes; and both readers — the hub type's
[Authorize] and the /alarms endpoint metadata — carry it.

SecretsNavRenderTests: registers the new policy (an unregistered policy name throws
rather than denying in an AuthorizeView) and gains the Alarms-link negative/control
pair.

Verified: dotnet build src/ZB.MOM.WW.MxGateway.NonWindows.slnx (zero warnings);
filtered run of the touched classes — 92 passed, 0 failed.

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

@@ -33,6 +33,18 @@ public static class DashboardAuthenticationDefaults
     /// </summary>
     public const string HubClientsPolicy = "MxGateway.Dashboard.HubClients";
 
+    /// <summary>
+    /// Policy guarding the alarm feed — the <c>AlarmsHub</c> SignalR surface and the
+    /// <c>/alarms</c> page that reads the same central monitor in process (SEC-40). Accepts the
+    /// same two schemes as <see cref="HubClientsPolicy"/> but carries
+    /// <see cref="AuthenticatedDashboardRoleRequirement"/>, a requirement type
+    /// <see cref="DashboardAuthorizationHandler"/> does not handle, so the environment bypasses
+    /// (<c>Authentication:Mode=Disabled</c> and anonymous localhost) cannot satisfy it. The alarm
+    /// feed is galaxy-wide and session-less, so unlike a session event stream there is no tag
+    /// intersection to narrow it with — requiring a real role claim is the available narrowing.
+    /// </summary>
+    public const string AlarmsPolicy = "MxGateway.Dashboard.Alarms";
+
     public const string LdapGroupClaimType = "mxgateway:ldap_group";
     public const string KeyPrefixClaimType = "mxgateway:key_prefix";
 
@@ -40,9 +52,13 @@ public static class DashboardAuthenticationDefaults
     /// Claim carrying one dashboard event-visibility tag the caller is granted (SEC-25). Stamped
     /// at cookie login by <see cref="DashboardAuthenticator"/> and at hub-token mint by
     /// <see cref="HubTokenService"/>, both resolving the caller's LDAP groups through
-    /// <c>MxGateway:Dashboard:GroupToTag</c>; read by <see cref="IDashboardSessionAcl"/>. A
-    /// principal carrying none of these claims is an empty-grant Viewer, which is the fail-closed
-    /// default. Visibility only — it never grants data access.
+    /// <c>MxGateway:Dashboard:GroupToTag</c>. Since SEC-41 <see cref="IDashboardSessionAcl"/> reads
+    /// this claim only as a FALLBACK, for a principal that carries no
+    /// <see cref="LdapGroupClaimType"/> claims to re-resolve from (the hub-token principal, whose
+    /// payload carries tags but not groups); a cookie principal is re-resolved against current
+    /// options instead, so a stamped tag cannot outlive a config-revoked grant. A principal
+    /// carrying neither kind of claim is an empty-grant Viewer, which is the fail-closed default.
+    /// Visibility only — it never grants data access.
     /// </summary>
     public const string DashboardTagClaimType = "zb:dashboardtag";