feat(ui): the search palette, entry points and a trail that survives the URL (CG-45)

Search: `/` or ⌘K focuses the box; results arrive grouped by kind with their
glyph, signature and file:line, ↑/↓/Enter walk them, Esc dismisses. A group
appears where its best result did, so flattening the groups reproduces the
ranking the keyboard walks — the panel's flat item list IS that concatenation.
A flow question ("how does X reach Y", "X -> Y") is recognised and searches
both endpoints with a note, rather than offering a row that would land on the
phase-2 Flow view.

Entry points answer "where do I start" on the empty screen and in the resting
palette, all derived from the graph: routes, files that run something at module
level (the engine records a top-level statement as an edge out of the file node,
which is what makes src/bin/codegraph.ts the root of the CLI flow — ranked by
calls x the files they reach, so a registration table calling into itself does
not outrank the CLI), and the most depended-on symbols. Tests are excluded from
both derived lists.

Trail: hops record the direction they were walked (→ into a call, ← up to a
caller), clicking one truncates back to it, Clear keeps the place instead of
throwing it away, and the whole walk travels in the URL. A shared or reloaded
trail arrives as ids, so hops learn their names back through a new batch
endpoint and a session name cache — without it, walking back across a
truncation redrew earlier hops as raw hashes. "Read as flow" stays hidden until
there is a Flow view to send it to.

New endpoints: /api/entrypoints and /api/nodes. New engine reads:
getTopCallingFiles, getFileDependentCounts.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

ui/src/App.svelte

@@ -9,11 +9,9 @@
   import FlowView from './views/FlowView.svelte';
   import NotFoundView from './views/NotFoundView.svelte';
   import { router, navigate, back, mapHref, flowHref } from './lib/router.svelte';
-  import { trail } from './lib/trail.svelte';
+  import { trail, resolveTrailNames } from './lib/trail.svelte';
   import { project } from './lib/project.svelte';
 
-  let query = $state('');
-
   // One `/api/stats` for the whole app: the top bar's counts and the Symbol
   // view's blast-radius denominator come out of the same payload.
   $effect(() => {
@@ -37,6 +35,14 @@
     });
   });
 
+  // Hops restored from a URL carry ids and nothing else; one batched request
+  // turns the bar back into names. Runs after every trail change, and does
+  // nothing when every hop already has one.
+  $effect(() => {
+    void trail.hops.length;
+    void resolveTrailNames();
+  });
+
   function isTypingTarget(target: EventTarget | null): boolean {
     if (!(target instanceof HTMLElement)) return false;
     return (
@@ -84,7 +90,7 @@
 
 <svelte:window {onkeydown} />
 
-<TopBar bind:this={topbar} bind:query project={project.name} stats={project.summary} />
+<TopBar bind:this={topbar} project={project.name} stats={project.summary} />
 <TrailBar />
 <main>
   {#if route.view === 'symbol'}