Implement CE-WP-0005 T01-T08: demo app — sessions, uploads, ZIP archive

Turn the MVP into a self-contained demo. Users now:
  1. Land on an empty-state and create a named session.
  2. Drag-drop or pick arbitrary PDFs into that session.
  3. Annotate, build evidence, link to form fields — all session-scoped.
  4. Export the whole session as a single .zip archive (manifest +
     per-document PDFs).
  5. Import a .zip back — into a new session, or merged into an
     existing one (documents deduped by SHA-256 fingerprint;
     annotations/evidence/links added additively).

Architecture:
- New shared types: SessionId, Session, SessionArchiveManifest +
  parseSessionArchiveManifest with schema-version validation.
- SessionService (engine/services/sessions.ts) handles lifecycle
  (create/rename/delete/setActive) + emits 4 new events through its
  own bus; SharedContracts.md §4 lists the additions.
- SessionProvider (work/SessionContext.tsx) owns the cross-session
  state: service, per-session PdfByteStore registry, per-session
  version counter that drives EngineProvider remounts after imports.
- EngineProvider becomes session-aware (sessionId prop drives per-
  session localStorage keys). Bumping engineRevision after
  restoreFromStorage forces consumers to re-render so restored repos
  show up immediately.
- PdfByteStore (source/pdf/byte-store.ts) holds Uint8Array bytes per
  document and mints blob URLs; ingestPdfFromFile is the upload
  entry-point that wraps the existing ingestPdf pipeline.
- ADR-0008 locks the ZIP layout (manifest.json + documents/<id>.pdf),
  the manifest schema (schemaVersion 1), and the merge-on-collision
  policy. JSZip is the only new dependency.
- App.tsx restructured: SessionProvider at the root, EngineProvider
  keyed by ${sessionId}:${version}, hash routing #/s/<id>[/forms/demo],
  SessionMenu top-bar, CreateFirstSession empty state.
- New DocumentRemoved event for per-document delete cleanup in
  CollectionList; engine.documents.remove() is the new service method.

Tests:
- Unit: 16 SessionService lifecycle + persistence tests;
  per-session snapshot round-trip; PdfByteStore + ingestPdfFromFile;
  SessionArchive parser; exportSessionZip + importSessionZip with
  create + merge + corrupt-archive paths.
- DOM: UploadDropzone, session-scoped CollectionList delete,
  SessionMenu create/switch/rename, routing parser.
- E2E: tests/integration/session-export-reimport.dom.test.tsx walks
  the full create → annotate → export → reimport flow and asserts
  the additive merge (deduped doc + doubled evidence rows).
- Legacy E2Es updated to use a seed-session helper instead of the
  removed fixture-button flow.

Known limitation (documented in ADR-0008): re-importing your own
freshly-exported ZIP creates duplicate annotations. Forward pointer
left for an importBundleId follow-up.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/engine/repos/in-memory.ts

@@ -25,12 +25,14 @@ export interface DocumentRepository {
   get(id: DocumentId): Document | null;
   list(): readonly Document[];
   update(document: Document): Document;
+  delete(id: DocumentId): boolean;
 }
 
 export interface RepresentationRepository {
   create(representation: DocumentRepresentation): DocumentRepresentation;
   get(id: RepresentationId): DocumentRepresentation | null;
   listByDocument(documentId: DocumentId): readonly DocumentRepresentation[];
+  deleteByDocument(documentId: DocumentId): number;
 }
 
 export interface AnnotationRepository {
@@ -82,6 +84,9 @@ export function createInMemoryRepos(): InMemoryRepos {
         documents.set(document.id, document);
         return document;
       },
+      delete(id) {
+        return documents.delete(id);
+      },
     },
     representations: {
       create(representation) {
@@ -98,6 +103,16 @@ export function createInMemoryRepos(): InMemoryRepos {
         }
         return out;
       },
+      deleteByDocument(documentId) {
+        let removed = 0;
+        for (const [id, rep] of representations) {
+          if (rep.documentId === documentId) {
+            representations.delete(id);
+            removed += 1;
+          }
+        }
+        return removed;
+      },
     },
     annotations: {
       create(annotation) {