§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/lib/entryPaths.js

@@ -1,22 +1,29 @@
-// §22.10 — project-scoped path builders. After M3 every entry/proposal link
-// lives under `/p/<project>/…`. Until Plan B serves multiple corpora, that
-// project id is the deployment's corpus-served default for chrome surfaces, or
-// the contextual project when a component renders inside a project subtree
-// (ProjectContext). Components build links via these helpers so the later
-// per-project-serving slice flips them in one place.
+// §22 three-tier — project + collection-scoped path builders. The canonical
+// entry route now carries the collection segment: `/p/<project>/c/<collection>/…`.
+// In S1 each project has a single (default) collection and serving stays
+// project-scoped, so the builders emit the default collection segment; the
+// collection-aware link layer (named collections) lands in S2. Components build
+// links via these helpers so that flip happens in one place.
 import { useProject } from '../components/ProjectLayout.jsx'
 import { useDeployment } from '../context/DeploymentProvider'
 
-export function entryPath(pid, slug) {
-  return `/p/${pid}/e/${slug}`
+// The default collection id (migration 029 seeds one per project at this id).
+export const DEFAULT_COLLECTION = 'default'
+
+export function entryPath(pid, slug, cid = DEFAULT_COLLECTION) {
+  return `/p/${pid}/c/${cid}/e/${slug}`
 }
 
-export function entryPrPath(pid, slug, prNumber) {
-  return `/p/${pid}/e/${slug}/pr/${prNumber}`
+export function entryPrPath(pid, slug, prNumber, cid = DEFAULT_COLLECTION) {
+  return `/p/${pid}/c/${cid}/e/${slug}/pr/${prNumber}`
 }
 
-export function proposalPath(pid, prNumber) {
-  return `/p/${pid}/proposals/${prNumber}`
+export function proposalPath(pid, prNumber, cid = DEFAULT_COLLECTION) {
+  return `/p/${pid}/c/${cid}/proposals/${prNumber}`
+}
+
+export function collectionHome(pid, cid = DEFAULT_COLLECTION) {
+  return `/p/${pid}/c/${cid}/`
 }
 
 export function projectHome(pid) {