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/symbol/SymbolHeader.svelte

@@ -94,11 +94,6 @@
       hub · {plural(payload.counts.callers, 'caller')}
     </span>
   {/if}
-  {#if payload.drift}
-    <span class="badge warn" title="The line ranges below come from the last index sync">
-      <span class="sw"></span>changed on disk after the last index sync
-    </span>
-  {/if}
   <span class="badge" class:warn={testBadge.warn} title={testBadge.title}>
     <span class="sw"></span>{testBadge.text}
   </span>