feat(modules): interleave module nav, derive moderator confinement

Phase 2, PR 8 of docs/website/MODULE_SYSTEM.md 2.7 - the nav half PR 7
deferred, plus the two seams 1.4 and 1.5 asked for.

withModuleNav (client/src/modules/nav.js) merges an installed module's rows
into core's three navs BEFORE the admin-override merge, and that ordering is
the design. applyNavOverrides and buildPublicNav are keyed by `to` and drop
any key their base array does not declare, so rows appended after the merge
would be unorderable, unrelabellable and unhideable in Admin - Navigation.
Today's UO rows are all three of those things, so appending would make the
extraction a visible regression for anyone who has ever edited their nav.
Merging first means a module row is an ordinary row downstream: nothing in
navOverrides.js, NavEditor.jsx or the layouts knows a module exists.

MOD_PATHS is gone. Moderator visibility and the redirect that confines a
moderator both derive from each row's own `roles`, in the new plain-JS
lib/adminNav.js (plain so the DOM-less runner can reach it). Two rows move,
both toward what the server already permitted: Dashboard, whose roles had
always named moderator, and My Characters, which is ungated self-service.

That also fixes a defect predating the module system. The redirect was a
THIRD hardcoded list - three path prefixes against MOD_PATHS' five paths -
and they disagreed about /admin/houses, so a moderator who clicked Houses in
their own sidebar was bounced back to Moderation. The derived allow-list is
computed from the BASE nav, never the override-merged one: an override is
presentation and must not move an authorization boundary either way.

The feature seam (modules/features.jsx + modules/featureGate.js) resolves a
row's `feature` against the provider its OWN module registered, so the
namespace comes from the registration and no string carries a parsed prefix.
Core registers useShardFlags under the owner id `core` - the client twin of
registries.registerCore() - so the ten shard-gated header rows already run
through the seam and Phase 3 deletes a registration instead of rewriting
SiteHeader. Every unknown fails open: no provider, a null answer while a
fetch is in flight, or a junk return all show the link, because the server is
the gate and hiding a page from someone entitled to it is the worse mistake.

933 server tests (unchanged - this PR is client-only), 160 client tests
(+37). routes.manifest.json unchanged at 230 routes; the OpenAPI spec
regenerates byte-identical.

Re-ran the MODULE_API.md 7.7 browser smoke, since this is the seam that rule
exists for. A throwaway module registering nav in all three areas and a
provider granting one flag and withholding another: the row lands inside
core's Moderation group rather than an appended block, the withheld row does
not render, a moderator reaches both /admin/houses and the module's admin
page, and an admin can relabel a module row and have it persist and apply.
Zero CSP reports, zero console errors.

Co-Authored-By: Claude <noreply@anthropic.com>

client/src/routes/player/PlayerPortalLayout.jsx

@@ -6,6 +6,8 @@ import { useAuth } from '../../contexts/AuthContext.jsx'
 import { useSite } from '../../contexts/SiteContext.jsx'
 import { applyNavOverrides } from '../../lib/navOverrides.js'
 import { useNavOverrides } from '../../lib/useNavOverrides.js'
+import { withModuleNav } from '../../modules/nav.js'
+import { useFeatureGate } from '../../modules/features.jsx'
 
 // Shared shell for the logged-in player portal. Uses the same sidebar shell as
 // Admin (icon nav, sticky content header, footer sign-out) so the two logged-in
@@ -35,9 +37,10 @@ const IconGear = () => <Icon><circle cx="12" cy="12" r="3" /><path d="M12 2v3M12
 const IconShield = () => <Icon><path d="M12 3l7 3v5c0 5-3.5 8-7 10-3.5-2-7-5-7-10V6z" /><path d="M9 12l2 2 4-4" /></Icon>
 
 // Exported because Admin -> Navigation edits this list. It stays declared here;
-// the editor may only relabel, reorder and hide what it finds (§7). No row
-// carries a gate — every player sees all three — so the merged result is what
-// renders, with no filter after it.
+// the editor may only relabel, reorder and hide what it finds (§7). No CORE row
+// carries a gate — every player sees all three — but an installed module's rows
+// join this list before the merge and may carry a `feature`, so the filter after
+// it is not dead code.
 export const NAV = [
   { to: '/player', label: 'Characters', end: true, icon: IconUser },
   { to: '/account/appeals', label: 'Appeals', icon: IconShield },
@@ -69,7 +72,12 @@ export default function PlayerPortalLayout() {
   const { user, logout } = useAuth()
   const { siteTitle } = useSite()
   const navOverrides = useNavOverrides()
-  const nav = useMemo(() => applyNavOverrides(NAV, navOverrides.nav_player), [navOverrides.nav_player])
+  const isVisible = useFeatureGate()
+  const baseNav = useMemo(() => withModuleNav(NAV, 'player'), [])
+  const nav = useMemo(
+    () => applyNavOverrides(baseNav, navOverrides.nav_player).filter(isVisible),
+    [baseNav, navOverrides.nav_player, isVisible],
+  )
   const navigate = useNavigate()
   const location = useLocation()
   const title =