feat(ui): live refresh and drift banners — the viewer keeps up with the project (CG-53)

`GET /api/events` is a server-sent-event stream the viewer holds open for the
life of the page. Two signals, two things the browser could not know:

  changed  source files touched on disk, before any sync — the drift banner
  index    the graph moved, naming what the sync re-indexed — the live refresh

The server WATCHES and never syncs: the project tree through the engine's own
FileWatcher with a notify-only syncFn, the index through one non-recursive
fs.watch on the data directory settled at 400 ms. Both start with the first
subscriber and stop with the last, so a viewer nobody has open costs no watch
descriptors. Nothing polls, on either side.

Drift is now parity with codegraph_node (#1474) rather than an absence.
`/api/source?ondrift=current` serves a drifted file's CURRENT bytes flagged
`showing: 'current'`, and the three screens that can say so switch off
everything anchored to the old line numbering — gutter ports, call-site links,
call arcs, the callee rail's anchoring — while keeping the source. The banner is
paper-2 with a hairline rule, never amber: amber belongs to the untested badge.

Also fixes a stale read this exposed. A long-lived reader holds an LRU of nodes
by id that only its own writes invalidate, so `/api/node/<id>` kept answering
with a symbol another process's sync had deleted while `/api/search` beside it
said it was gone. GraphSession now drops the read caches when the database (or
its WAL) has been written, and the Symbol view follows a symbol whose id changed
because an edit above it moved its start line, carrying the trail across.

Measured on a live viewer: banner 360 ms after a save, toast 440 ms after
`codegraph sync` returns, 0 requests in 4 idle seconds, and the client gives up
reconnecting after ~90 s with "Not live" rather than hammering a dead port.

ui/src/views/FlowView.svelte

@@ -19,6 +19,7 @@
   import FlowCard from '../components/flow/FlowCard.svelte';
   import FlowLink from '../components/flow/FlowLink.svelte';
   import { fetchFlow, type WireFlow, type WireFlowPayload } from '../lib/api';
+  import { live } from '../lib/live.svelte';
   import { navigate, symbolHref } from '../lib/router.svelte';
   import { trail, encodeTrail, type TrailHop } from '../lib/trail.svelte';
   import { decodeTrail } from '../lib/trail-codec';
@@ -72,14 +73,19 @@
       error = null;
       return;
     }
+    // Re-run when the index moves: a path is a walk over edges that a sync can
+    // add, remove or re-route, and a strip drawn from the previous graph would
+    // disagree with `codegraph_explore` about the same question.
+    void live.indexTick;
     const controller = new AbortController();
     loading = true;
     error = null;
+    const keep = picked;
     fetchFlow(spec, controller.signal)
       .then((next) => {
         payload = next;
-        picked = next.flows[0]?.id ?? null;
-        showAll = false;
+        // A refresh keeps the reader's chosen path when it survived the sync.
+        picked = next.flows.some((f) => f.id === keep) ? keep : (next.flows[0]?.id ?? null);
         loading = false;
       })
       .catch((err: unknown) => {