§22 S1: frontend /c/<collection>/ route layer + C3.7 + legacy-URL redirects

- entryPaths builders carry the /c/<collection>/ segment (default collection in S1)
- /p/:projectId/* gains c/:collectionId/ corpus routes; serving stays project-scoped
- DefaultCollectionRedirect (C3.7: project landing -> default collection)
- LegacyCorpusRedirect (v0.35.0 /p/<p>/e/<slug> bookmarks -> /c/default/, query preserved)
- entryPaths unit test; build + vitest green (18 tests)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/App.jsx

@@ -1,10 +1,10 @@
 import { useEffect, useRef, useState } from 'react'
-import { Routes, Route, Link, Navigate, useLocation, useNavigate, useSearchParams } from 'react-router-dom'
+import { Routes, Route, Link, Navigate, useLocation, useNavigate, useParams, useSearchParams } from 'react-router-dom'
 import { getMe, subscribeToNotifications } from './api'
 import { anonymize, EVENTS, identify, track } from './lib/analytics'
 import { useLastState } from './lib/useLastState'
 import { brandTitle } from './lib/brand'
-import { entryPath, proposalPath } from './lib/entryPaths'
+import { entryPath, proposalPath, DEFAULT_COLLECTION } from './lib/entryPaths'
 import { useDeployment } from './context/DeploymentProvider'
 import ProjectLayout from './components/ProjectLayout.jsx'
 import Directory from './components/Directory.jsx'
@@ -360,10 +360,21 @@ export default function App() {
               />
               <main className="main-pane">
                 <Routes>
-                  <Route path="" element={<Welcome viewer={viewer} />} />
-                  <Route path="e/:slug" element={<RFCView viewer={viewer} />} />
-                  <Route path="e/:slug/pr/:prNumber" element={<PRView viewer={viewer} />} />
-                  <Route path="proposals/:prNumber" element={<ProposalView viewer={viewer} onChange={() => setCatalogVersion(v => v + 1)} />} />
+                  {/* §22 three-tier (C3.7): the project landing redirects into
+                      its sole/default collection (S1: the `default` one). */}
+                  <Route path="" element={<DefaultCollectionRedirect />} />
+                  {/* Backcompat: the shipped v0.35.0 corpus URLs without a
+                      /c/<collection>/ segment redirect into the default
+                      collection, so old bookmarks keep working. */}
+                  <Route path="e/:slug" element={<LegacyCorpusRedirect kind="entry" />} />
+                  <Route path="e/:slug/pr/:prNumber" element={<LegacyCorpusRedirect kind="entryPr" />} />
+                  <Route path="proposals/:prNumber" element={<LegacyCorpusRedirect kind="proposal" />} />
+                  {/* Collection-scoped corpus. Serving stays project-scoped in
+                      S1 (collection = default); collection-aware serving is S2. */}
+                  <Route path="c/:collectionId" element={<Welcome viewer={viewer} />} />
+                  <Route path="c/:collectionId/e/:slug" element={<RFCView viewer={viewer} />} />
+                  <Route path="c/:collectionId/e/:slug/pr/:prNumber" element={<PRView viewer={viewer} />} />
+                  <Route path="c/:collectionId/proposals/:prNumber" element={<ProposalView viewer={viewer} onChange={() => setCatalogVersion(v => v + 1)} />} />
                 </Routes>
               </main>
             </ProjectLayout>
@@ -403,6 +414,28 @@ export default function App() {
   )
 }
 
+// §22 three-tier — corpus redirects mounted under /p/:projectId/*.
+// C3.7: the project landing skips the (single-collection) directory and lands in
+// the project's default collection.
+function DefaultCollectionRedirect() {
+  const { projectId } = useParams()
+  return <Navigate to={`/p/${projectId}/c/${DEFAULT_COLLECTION}/`} replace />
+}
+
+// Backcompat for the shipped v0.35.0 corpus URLs that lacked the
+// /c/<collection>/ segment: redirect into the default collection, preserving any
+// query string (e.g. ?branch=).
+function LegacyCorpusRedirect({ kind }) {
+  const { projectId, slug, prNumber } = useParams()
+  const { search } = useLocation()
+  const base = `/p/${projectId}/c/${DEFAULT_COLLECTION}`
+  let to = `${base}/`
+  if (kind === 'entry') to = `${base}/e/${slug}`
+  else if (kind === 'entryPr') to = `${base}/e/${slug}/pr/${prNumber}`
+  else if (kind === 'proposal') to = `${base}/proposals/${prNumber}`
+  return <Navigate to={to + (search || '')} replace />
+}
+
 function DeploymentLanding() {
   // §22.10 + design decision 2 — N=1 lands in the single visible project so
   // OHM's "land in the corpus" UX is preserved; the directory appears only