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.

CHANGELOG.md

@@ -36,6 +36,16 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
   In the left margin is an arc for every call that stays inside the file, drawn from the calling line to the line the callee is defined on. Nothing is laid out by an algorithm — the author already put the symbols in order, so source order does the work, and this is the one place a file's internal call structure is legible at a glance. Hover a line to light the arcs the function under your cursor takes part in, and click an arc to jump to the other end. On a file with more than forty of them the picture narrows to the symbol you're reading instead of drawing a wash of overlapping sweeps, with the total in the header. A rail on the far left lists the file's symbols and follows you as you scroll, when the window is wide enough for it.
 
+- **The viewer keeps up with your project while it's open.** Save a file and `codegraph ui` says so within about a third of a second: a banner on the file's screen explaining that the index hasn't caught up yet, and the file's **current** source in place of a body sliced at line numbers it no longer has — the call arcs, gutter markers and call list go away with the old numbering rather than pointing at the wrong lines. It's the same answer `codegraph_node` gives your agent about a file that changed after its last sync.
+
+  When anything re-indexes the project — your agent's background sync, `codegraph sync`, a git hook — whatever is on screen re-reads the graph and a small "Index updated · reloaded" note appears. A symbol that moved because you added a line above it is followed to its new place, with your trail intact, instead of turning into a dead link.
+
+  Nothing polls: the viewer watches for these two things and is told about them. If it loses touch with the server it retries a few times with a growing delay, then stops and says "Not live" in the top bar rather than hammering a port that isn't answering.
+
+### Fixes
+
+- Fixed a long-running `codegraph ui` session serving a symbol that a sync had already deleted. The viewer keeps one connection to your index open, and its in-memory lookup didn't notice when another process — your agent's sync, or `codegraph sync` — rewrote the file underneath it, so a symbol screen could keep showing a body with no callers while search correctly reported it had moved. Because a symbol's identity includes the line it starts on, this happened after almost any edit above it.
+
 
 ## [1.6.0] - 2026-08-26