test(dashboard)+docs: SEC-25 live-LDAP ACL coverage; design marked implemented

The per-session dashboard event ACL shipped in 693a78d + 7ec0b35 with unit
coverage over a fabricated principal. What a fabricated principal cannot show is
that the group names the shared directory actually returns -- short RDN values,
not DNs -- are the ones Dashboard:GroupToTag keys match. Two [LiveLdapFact]s
close that: gw-viewer binds for real, its GwReader membership grants team-a, and
IDashboardSessionAcl then admits a team-a-tagged session and refuses a
team-b-tagged one; multi-role takes the Administrator bypass. The mapping is
config-side only -- no GLAuth entry, group, or membership was added, and
glauth.md records that explicitly so a future reader does not go looking for a
directory change that never happened.

multi-role is a member of GwReader as well as GwAdmin, so it holds team-a too.
Its bypass is therefore asserted on team-b and on the untagged session -- the two
it would lose if the Administrator branch were ever dropped -- rather than on
team-a, which would pass either way.

One cheap hardening from a prior review: a GatewayOptionsTests case binds
Dashboard:GroupToTag through a real ConfigurationBuilder and looks the group up
mis-cased. The property initializer seeds an OrdinalIgnoreCase dictionary, but
only the binder decides whether that instance survives; if it did not, a
mis-cased group name from the directory would grant no tags and the ACL would
deny with no diagnostic.

Docs follow the shipped shape: docs/Sessions.md gains the session-tag model
(owner-key sourced, immutable, visibility-not-access), gateway.md and CLAUDE.md
gain the ACL in their dashboard-auth paragraphs, and three
GatewayDashboardDesign.md passages that still described the ACL as outstanding
now describe both gated seams and the decision order. GatewayConfiguration.md's
ShowTagValues row no longer claims the redaction is the only thing between a
Viewer and another session's values -- it is now the second of two independent
layers. gateway.md's hub-token lifetime corrected 30 minutes -> 5, matching
HubTokenService. Authentication.md disambiguates --dashboard-tags as the only
constraint flag that splits on commas. The plan doc header is Implemented; its
as-built section 12 already existed and is not duplicated.

Verified: NonWindows.slnx builds clean; GatewayOptions/DashboardSessionAcl/
EventsHub filters 37/37; the live-LDAP suite skips cleanly without the env var
and runs 7/7 green against the shared GLAuth with it.

docs/GatewayConfiguration.md

@@ -192,7 +192,7 @@ events (a "gap") and must re-snapshot; whatever is still retained is replayed.
 | `MxGateway:Dashboard:SnapshotIntervalMilliseconds` | `1000` | Dashboard snapshot refresh interval used by the snapshot SignalR hub and the pages that subscribe to it. |
 | `MxGateway:Dashboard:RecentFaultLimit` | `100` | Maximum number of fault summaries projected into each dashboard snapshot. |
 | `MxGateway:Dashboard:RecentSessionLimit` | `200` | Maximum number of session summaries projected into each dashboard snapshot. |
-| `MxGateway:Dashboard:ShowTagValues` | `false` | Controls whether tag values reach the dashboard's SignalR events hub mirror. `false` (default): `DashboardEventBroadcaster` blanks tag values from a deep-cloned copy of each `MxEvent` before it reaches any hub subscriber — event metadata (tag reference, quality, status, timestamps) still renders; see `docs/GatewayDashboardDesign.md`'s `EventsHub` row for the mechanism. Security-relevant because the per-session hub ACL that would scope a Viewer to specific sessions does not exist yet: with no per-session scoping, this redaction is currently the only thing standing between a low-trust Viewer and other sessions' tag values, so setting this `true` exposes every session's tag values to every authenticated dashboard viewer. The flag gates only the SignalR hub mirror — it does **not** cover the `/browse` live-value display, which remains a separate, still-open residual. |
+| `MxGateway:Dashboard:ShowTagValues` | `false` | Controls whether tag values reach the dashboard's SignalR events hub mirror. `false` (default): `DashboardEventBroadcaster` blanks tag values from a deep-cloned copy of each `MxEvent` before it reaches any hub subscriber — event metadata (tag reference, quality, status, timestamps) still renders; see `docs/GatewayDashboardDesign.md`'s `EventsHub` row for the mechanism. This is now the second of two independent layers, not the only one: `IDashboardSessionAcl` decides *which* sessions a caller may subscribe to at all (see `GroupToTag` / `UntaggedSessionVisibility` below), while this flag decides what a permitted subscriber sees. Setting it `true` therefore exposes tag values to everyone the ACL admits — every Administrator, plus each Viewer holding a matching tag. The flag gates only the SignalR hub mirror — it does **not** cover the `/browse` live-value display, which remains a separate, still-open residual. |
 | `MxGateway:Dashboard:GroupToRole` | _(empty)_ | LDAP group → dashboard role mapping. Keys are LDAP group names (short CN or full DN — leading-RDN match). Values must be `Admin` (read/write, API-key CRUD) or `Viewer` (read-only). A user whose LDAP groups don't intersect this map cannot sign in; with no mapping at all, only the loopback bypass admits anyone. |
 | `MxGateway:Dashboard:GroupToTag` | _(empty)_ | LDAP group → dashboard visibility tags. Keys follow the same convention as `GroupToRole` (short CN or full DN — leading-RDN match, case-insensitive); values are tag lists. A dashboard user's granted tag set is the union over the groups they belong to; an unmapped group contributes nothing. **Visibility only:** tags scope which sessions' event streams a Viewer may observe on the dashboard — they never grant or deny data access, which stays with the API key's scopes and constraints. Independent of `GroupToRole`: a group may appear in either map, both, or neither. Empty (the default) means Viewers hold no tags, so under the default `UntaggedSessionVisibility` they observe no session's events. |
 | `MxGateway:Dashboard:UntaggedSessionVisibility` | `AdminOnly` | Who may observe a session that carries no tags (its owning API key declared none). `AdminOnly` (default, fail-closed) restricts untagged sessions to dashboard Administrators. `AllViewers` shows them to every Viewer — opt-in for a single-tenant deployment that wants the pre-tag behaviour. Administrators always see every session regardless of tags. |