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/README.md

@@ -45,7 +45,9 @@ src/
   lib/map-model.ts        the Map's deterministic layered layout (pure)
   lib/flow-model.ts       the Flow strip's card/link geometry — a DAG (pure)
   lib/filecode-model.ts   the whole-file view: fixed line height, arcs, paging (pure)
-  components/             TopBar, TrailBar, KindGlyph, map/, flow/, symbol/, file/
+  lib/live.svelte.ts      /api/events: two counters every screen refreshes from
+  lib/toast.svelte.ts     the one transient note ("Index updated · reloaded")
+  components/             TopBar, TrailBar, KindGlyph, DriftBanner, Toast, map/, flow/, symbol/, file/
   views/                  one component per route
 ```
 
@@ -66,6 +68,29 @@ announce the project to a font CDN.
 | `#/flow?symbols=a,b,c` | flow strip — `codegraph_explore`'s own question |
 | `#/flow?t=<trail>` | flow strip — the trail you walked, read as a flow |
 
+## Live updates
+
+The viewer never polls. `lib/live.svelte.ts` holds one `EventSource` on
+`/api/events` for the life of the page and exposes two counters:
+
+- **`indexTick`** — the graph moved (somebody synced). Every screen refetches:
+  a rail is an answer about the whole graph, and a symbol gains a caller when
+  some *other* file is edited, so filtering by the focused file would leave the
+  rails quietly wrong. One request per sync.
+- **`diskTick`** — source files changed on disk and the index has not caught up.
+  Only the screen showing one of those files reacts, and what it does is draw a
+  drift banner.
+
+`liveRefresh(file, refresh)` is the three lines of bookkeeping that turns a
+counter into a single call; the Map and the Flow strip instead read
+`live.indexTick` straight inside the effect that already fetches them.
+
+Reconnection is ours, not `EventSource`'s: each failure closes the stream and
+schedules ONE retry on a backoff that ends after eight attempts (~90 s), at
+which point the top bar says "Not live" and nothing more is requested until the
+tab is focused again. A `degraded` event — the server's watcher gave up — is
+shown the same way and never answered with a poll.
+
 Node ids and file paths are encoded per slash-separated segment, so
 `#/file/src/mcp/tools.ts` stays readable and still round-trips a segment
 containing a reserved character. Build hashes with `symbolHref()` /