feat(sessions): stream in-flight partial when joining a mid-turn session

Seed the in-flight partial assistant message (text, thinking, and
in-progress tool calls) when opening or reconnecting to a session that is
mid-stream, then continue streaming live deltas on top of it. Replaces the
blocking "Catching up..." placeholder and the end-of-turn transcript reload.

Server stamps every per-session UI event with a monotonic seq at the
SessionEventHub publish choke point and exposes
GET /sessions/:sessionId/stream-snapshot returning { seq, partial }. The
client fetches the snapshot on join, seeds the normalized partial into the
in-memory transcript (never the history cache), and applies buffered/live
events using the seq watermark for exactly-once delivery.

The snapshot is a progressive enhancement: a 404 from an older remote
pi-web or a not-yet-restarted session daemon falls back to an empty seed
(seq 0, drops nothing), so sessions still open and stream normally. The
stream-snapshot route is registered in the federation allowlist for
remote-machine proxying.

src/server/sessions/sessionRoutes.ts

@@ -99,6 +99,14 @@ export function registerSessionRoutes(app: FastifyInstance, sessions: SessionRou
     }
   });
 
+  app.get<{ Params: { sessionId: string }; Querystring: SessionQuery }>(`${prefix}/sessions/:sessionId/stream-snapshot`, async (request, reply) => {
+    try {
+      return await sessions.streamSnapshot(sessionLookupFromQuery(request.params.sessionId, request.query));
+    } catch (error) {
+      return reply.code(404).send({ error: errorMessage(error) });
+    }
+  });
+
   app.get<{ Params: { sessionId: string }; Querystring: SessionQuery }>(`${prefix}/sessions/:sessionId/models`, async (request, reply) => {
     try {
       return { models: await sessions.availableModels(sessionLookupFromQuery(request.params.sessionId, request.query)) };