feat: M3.5 polish — genre splitting + radio button surface

Two carry-overs from M3 verification, bundled with the search-input
fix already on dev (b7a59a9).

1. Genre splitting in BuildSessionVector
   The library has many tracks whose genre tag is a denormalized
   multi-genre string ("Indie Pop; Pop; Alternative Pop"). Reading them
   as one opaque tag means a single-genre "Pop" track and a multi-genre
   track listing Pop both fail to share the Pop key, so the tags axis
   in similarity scoring (weight 0.7!) returned 0 in nearly all cases
   on real libraries. Result: contextual scoring couldn't differentiate.

   Add splitGenres() that splits on ; and , and trims whitespace;
   strings without a delimiter come back as a single-element slice
   (so the existing single-genre cases keep working unchanged).
   Concatenated-without-separator output ("ElectronicComplextroGlitch
   Hop") stays opaque — that needs a genre dictionary, out of scope.

2. Play-radio button on TrackRow
   playRadio() was only wired to the click handler on /search and
   /search/tracks, leaving /library/liked, album pages, and search
   results' inner clicks with no way to start a radio. Adds a small
   radio button to TrackRow (between LikeButton and the +queue button).
   Click → playRadio(track.id), with stopPropagation so the row's own
   activate handler doesn't also fire.

Tests:
- 3 new sessionvector tests: TestSplitGenres, multi-genre semicolon
  split, no-separator stays opaque. Existing single-genre tests pass
  unchanged.
- 1 new TrackRow test: radio button calls playRadio with track id and
  does NOT trigger row play.

Verified locally: go -short -race ./... clean, golangci-lint clean,
svelte-check 0/0, 175 vitest tests (was 174), web build succeeds.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/lib/components/TrackRow.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
   import type { TrackRef } from '$lib/api/types';
   import { formatDuration } from '$lib/media/duration';
-  import { playQueue, enqueueTrack } from '$lib/player/store.svelte';
+  import { playQueue, enqueueTrack, playRadio } from '$lib/player/store.svelte';
   import LikeButton from './LikeButton.svelte';
 
   type PlayHandler = (tracks: TrackRef[], index: number) => void;
@@ -34,6 +34,11 @@
     e.stopPropagation();
     enqueueTrack(track);
   }
+
+  function onRadioClick(e: MouseEvent) {
+    e.stopPropagation();
+    playRadio(track.id);
+  }
 </script>
 
 <div
@@ -42,13 +47,20 @@
   aria-label={track.title}
   onclick={onRowClick}
   onkeydown={onRowKey}
-  class="grid w-full cursor-pointer grid-cols-[32px_1fr_auto_auto_auto] items-center gap-4 px-3 py-2 text-left text-sm odd:bg-surface/50 hover:bg-surface focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+  class="grid w-full cursor-pointer grid-cols-[32px_1fr_auto_auto_auto_auto] items-center gap-4 px-3 py-2 text-left text-sm odd:bg-surface/50 hover:bg-surface focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
 >
   <span class="text-right tabular-nums text-text-secondary">
     {track.track_number ?? '—'}
   </span>
   <span class="truncate">{track.title}</span>
   <LikeButton entityType="track" entityId={track.id} />
+  <button
+    type="button"
+    aria-label="Play radio from this track"
+    title="Play radio"
+    onclick={onRadioClick}
+    class="rounded p-1 text-text-secondary hover:text-text-primary"
+  >📻</button>
   <button
     type="button"
     aria-label="Add to queue"

web/src/lib/components/TrackRow.test.ts

@@ -5,7 +5,8 @@ import { readable } from 'svelte/store';
 
 vi.mock('$lib/player/store.svelte', () => ({
   playQueue: vi.fn(),
-  enqueueTrack: vi.fn()
+  enqueueTrack: vi.fn(),
+  playRadio: vi.fn()
 }));
 
 vi.mock('$lib/api/likes', () => ({
@@ -24,7 +25,7 @@ vi.mock('@tanstack/svelte-query', async (orig) => {
 });
 
 import TrackRow from './TrackRow.svelte';
-import { playQueue, enqueueTrack } from '$lib/player/store.svelte';
+import { playQueue, enqueueTrack, playRadio } from '$lib/player/store.svelte';
 
 const tracks: TrackRef[] = [
   {
@@ -91,4 +92,11 @@ describe('TrackRow', () => {
     expect(enqueueTrack).toHaveBeenCalledWith(tracks[0]);
     expect(playQueue).not.toHaveBeenCalled();
   });
+
+  test('radio button calls playRadio with track id and does NOT trigger row play', async () => {
+    render(TrackRow, { props: { tracks, index: 0 } });
+    await fireEvent.click(screen.getByRole('button', { name: /play radio/i }));
+    expect(playRadio).toHaveBeenCalledWith('t1');
+    expect(playQueue).not.toHaveBeenCalled();
+  });
 });