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/components/TopBar.svelte

@@ -5,6 +5,7 @@
   import SearchPalette from './SearchPalette.svelte';
   import type { PaletteItem } from '../lib/search-model';
   import { walkTo } from '../lib/walk';
+  import { live } from '../lib/live.svelte';
 
   interface Props {
     /** Indexed project name, e.g. "codegraph/". Null until stats load. */
@@ -111,6 +112,31 @@
   }
 
   let searchBox: HTMLDivElement | null = $state(null);
+
+  /**
+   * Why this page has stopped updating itself, when it has.
+   *
+   * The whole point of the live channel is that the screen keeps up with the
+   * project; a screen that has silently stopped keeping up is worse than one
+   * that never claimed to. So both ways it can end say so, in the one place
+   * that is on every view.
+   */
+  let liveNote = $derived.by(() => {
+    if (live.degraded !== null) {
+      return {
+        text: 'Live updates off',
+        title: `${live.degraded} This page no longer refreshes itself — reload it after a sync.`,
+      };
+    }
+    if (live.stopped) {
+      return {
+        text: 'Not live',
+        title:
+          'Lost the connection to codegraph ui and stopped retrying. Focus this tab to try again, or reload the page.',
+      };
+    }
+    return null;
+  });
 </script>
 
 <svelte:window {onpointerdown} />
@@ -152,6 +178,7 @@
   </div>
 
   <div class="project" title="Indexed project">
+    {#if liveNote}<span class="offline" title={liveNote.title}>{liveNote.text}</span>{/if}
     {#if project}<span class="mono">{project}</span>{/if}
     {#if stats}<span class="dim">{stats}</span>{/if}
   </div>
@@ -246,9 +273,20 @@
     white-space: nowrap;
   }
 
-  /* Below ~1000px the stats are the first thing worth losing. */
+  .offline {
+    padding: 2px 6px;
+    margin-right: 8px;
+    border: 1px solid var(--rule-soft);
+    background: var(--paper-2);
+    color: var(--ink-3);
+    font-size: 11.5px;
+  }
+
+  /* Below ~1000px the stats are the first thing worth losing — but not the
+     note that the page has stopped updating itself. */
   @media (max-width: 1000px) {
-    .project {
+    .project .mono,
+    .project .dim {
       display: none;
     }
   }