docs(nav): say why Insights stays closed under a switch, since the old reason lapsed

The entry carried "Non-delegable in v1, server-side and here". That was true at
v1.36.0 and is not true now: twenty-seven `/api/insights/*` reads plus
`/api/dashboard/summary` and `/api/export/health-record` declare
`("manage", "record")`, and the AI-egress objection they were closed for is
answered by construction rather than by the flag — the record resolver stamps
`setDelegatedGenerationSuppressed`, so a delegate reads what the owner's own
account generated and causes no egress.

What keeps the door shut is the overview's own reads rather than the tiles':
`/api/insights/layout` and `/api/insights/generate` both resolve `requireAuth()`
and refuse any switch at any level. Opening the entry without first deciding
what those two mean for a delegate — whose saved layout, and whether the
briefing reads or regenerates — would hand a whole-record MANAGE delegate a page
whose chrome 403s while its tiles answer, which is the failure the presentation
inventory exists to prevent. That is a decision about two more routes, not a
flag on this one, so the comment now says what is actually load-bearing and the
Coach's own reason is stated separately.

src/components/layout/nav-model.ts

@@ -224,11 +224,28 @@ export const NAV_DESTINATIONS: ReadonlyArray<NavDestination> = [
     tourId: "nav-documents",
     requiresModule: "inboundDocuments",
   },
-  // v1.36.0 — Insights and the Coach carry no `sharedRecord` flag, so they
-  // drop out under a switch. Both are AI surfaces, and AI egress of a
-  // person's health data rides the consent THAT person gave for their own
-  // use; a delegate triggering it would create a consent-shaped act the owner
-  // never made. Non-delegable in v1, server-side and here (design §4).
+  // Insights and the Coach carry no `sharedRecord` flag, so they drop out
+  // under a switch. The reason has changed since v1.36.0 and the old wording
+  // ("non-delegable, server-side and here") is no longer true of the tiles:
+  // twenty-seven `/api/insights/*` reads plus `/api/dashboard/summary` and
+  // `/api/export/health-record` declare `("manage", "record")`, and the
+  // AI-egress objection they were closed for is answered by construction —
+  // the record resolver stamps `setDelegatedGenerationSuppressed`, so a
+  // delegate reads what the owner's own account generated and causes no
+  // egress.
+  //
+  // What keeps the door shut is the OVERVIEW's own reads rather than the
+  // tiles': `/api/insights/layout` (`insights/layout/route.ts`) and
+  // `/api/insights/generate` (`insights/generate/route.ts`) both resolve
+  // `requireAuth()`, which refuses any switch at any level. Opening the entry
+  // without deciding what those two mean for a delegate — whose saved layout,
+  // and whether the briefing reads or regenerates — would hand a MANAGE
+  // delegate a page whose chrome 403s while its tiles answer. That is a
+  // decision about two more routes, not a flag on this one.
+  //
+  // The Coach stays closed on the original argument, unchanged:
+  // `/api/insights/chat` is genuinely not delegable, and the page is a
+  // generation surface rather than a read of one.
   {
     href: "/insights",
     tKey: "nav.insights",