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>

docs/design/codegraph-ui-design-spec.md

@@ -168,6 +168,20 @@ Results panel under the input: 1px `--ink` border, max-height 420px; group heade
 rows grid `18px | 1fr | auto`, 6px 10px, `--rule-faint` separators, selected/hover `--press`; name 12.5px mono + signature 11.5px mono
 `--ink-3` + location 11px mono. Flow grammar: "how does X reach Y", "X -> Y", "X → Y".
 
+**As built (phase 1, CG-45).** Group headers are the result's KIND — `Methods`, `Functions`,
+`Classes`, `Files` — a group appearing where its best result did, so flattening the groups
+reproduces the ranking ↑/↓ walks. The prototype's two-group split (`Flow` / `Symbols & files`)
+waits for the Flow view: a flow question is recognised now, but until there is a path to draw it
+searches both endpoints and says so in one line above the results rather than offering a row that
+lands on a placeholder. A file's row shows its basename with its DIRECTORY in the location column —
+its name column already carries the path, and printing it twice reads as an error.
+
+At rest — an empty box, or the empty screen — the panel shows **entry points** from
+`/api/entrypoints`: routes (URL → handler), files that run something at module level (a CLI, a
+worker entry, a script — ranked by calls × the number of other files they reach), and the most
+depended-on symbols. Each section says what it is derived from, never that a file IS the entry
+point.
+
 ## 4. Libraries and versions
 - Svelte 5 (≥ 5.25) + Vite (workspace `ui/`), Svelte Flow `@xyflow/svelte` ^1.6 for the Map and Flow canvases only (custom nodes/edges,
   hidden handles for port spreading, local selection state — the pattern in docker-app's `StackGraph.svelte`); `@dagrejs/dagre` only as a