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/modules/features.jsx

@@ -0,0 +1,65 @@
+import { createContext, useContext, useMemo, useState } from 'react'
+import { featureProviders } from './registry.js'
+import { buildFeatureGate, OPEN_GATE } from './featureGate.js'
+
+// The React half of the feature seam. The decision logic is featureGate.js,
+// which is plain JS and therefore testable in a runner with no DOM; this file is
+// wiring, the same split registry.js and shared.js already use.
+//
+// **Calling a hook per provider inside a loop is the point, and it is legal
+// here.** The rules of hooks require the same hooks in the same order on every
+// render of a component — not a statically known list. The provider list is
+// fixed before the first render (registration happens while module chunks
+// evaluate, and main.jsx does not mount until DOMContentLoaded), there is no
+// unregistering, and the snapshot below freezes it per component instance
+// anyway. So the loop's length cannot change between renders of this provider,
+// which is the actual requirement.
+//
+// A provider hook returns a Set-like of the flags this viewer may see, or `null`
+// while it is still fetching. Core knows nothing else about it: what a flag
+// means, how it is fetched, and what it is gated on are all the module's.
+
+const FeatureGateContext = createContext(OPEN_GATE)
+
+export function ModuleFeaturesProvider({ children }) {
+  // Snapshotted once. useState's initialiser runs on the first render only, so
+  // even a provider that somehow registered late cannot change this instance's
+  // hook count mid-life — it would be ignored until the next mount, which is a
+  // far better failure than a crashed render.
+  const [providers] = useState(featureProviders)
+
+  // eslint-disable-next-line react-hooks/rules-of-hooks -- fixed-length list, see above
+  const values = providers.map((provider) => provider.hook())
+
+  const gate = useMemo(
+    () => {
+      const byOwner = new Map()
+      // First registration wins for a given owner: a module that registers two
+      // namespaces answers its own nav rows from the first, rather than from
+      // whichever happened to be stored last.
+      providers.forEach((provider, i) => {
+        if (!byOwner.has(provider.id)) byOwner.set(provider.id, values[i])
+      })
+      return buildFeatureGate(byOwner)
+    },
+    // One dependency per provider — a fixed-length list, for the same reason the
+    // hook loop above is fixed-length.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+    [providers, ...values],
+  )
+
+  return <FeatureGateContext.Provider value={gate}>{children}</FeatureGateContext.Provider>
+}
+
+/**
+ * The predicate to filter nav rows with: `(item) => boolean`, true when the row
+ * carries no `feature` or when its module says this viewer may see it.
+ *
+ * Outside a provider it is the open gate, so a component rendered in isolation
+ * (a test, a preview) shows its whole nav rather than none of it.
+ */
+export function useFeatureGate() {
+  return useContext(FeatureGateContext)
+}
+
+export default ModuleFeaturesProvider