feat(play): pick same-name songs via !search / #N / id: / URL (#90)

!play/!add/!playnext only ever searched with limit 1, so a same-name song could
never be reached from chat (e.g. 'Die For You' always returned the most popular
match, not The Weeknd's). Add three disambiguation paths via a shared resolver:

- !search <name> — list the top matches (numbered, with id), remembered per bot
- !play #N / !add #N — play/queue the Nth result of the last !search
- !play id:<id> and pasted NetEase/QQ/BiliBili song URLs — play an exact song

Pure parsing (parseSongRef / parseSelectionIndex) is unit-tested; plain-text
search keeps the historical top-hit behavior. WebUI search (20 results) already
allowed picking same-name songs and is unchanged.

Fixes #90

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/bot/song-ref.ts

@@ -0,0 +1,68 @@
+/**
+ * Parsing helpers for picking an EXACT song in a !play / !add / !playnext query,
+ * so same-name songs can be disambiguated instead of always getting the single
+ * most-popular search hit (issue #90).
+ *
+ * Two mechanisms:
+ *  - A song reference: an explicit id / platform URL → play that exact song.
+ *  - A selection index: "#N" → the Nth result of the previous !search.
+ */
+
+export interface SongRef {
+  id: string;
+  /**
+   * Platform inferred from a URL. `null` means the platform wasn't encoded in
+   * the reference (e.g. a bare `id:`), so the caller should fall back to the
+   * command's flags / default provider.
+   */
+  platform: "netease" | "qq" | "bilibili" | null;
+}
+
+/**
+ * Detect an explicit song reference in a query. Recognizes:
+ *   - `id:<id>`                         → platform from flags/default
+ *   - NetEase song URL                  → music.163.com/song?id=N (also /#/song?id=N, /song/N)
+ *   - QQ song URL                       → y.qq.com/.../songDetail/MID (or ?songmid=MID)
+ *   - BiliBili BVID (bare or in a URL)  → bilibili.com/video/BVxxxx, b23.tv, or BVxxxx
+ * Returns `null` for a plain search term (the common case).
+ */
+export function parseSongRef(raw: string): SongRef | null {
+  const q = (raw ?? "").trim();
+  if (!q) return null;
+
+  // Explicit "id:<id>" — platform decided by the command's flags/default.
+  const idPrefix = /^id:\s*(\S+)$/i.exec(q);
+  if (idPrefix) return { id: idPrefix[1], platform: null };
+
+  // BiliBili BV id, bare or inside a bilibili URL (NetEase ids are numeric, so
+  // a "BV..." token never collides with them).
+  const bv = /BV[0-9A-Za-z]{8,12}/.exec(q);
+  if (bv && (/^BV[0-9A-Za-z]{8,12}$/.test(q) || /bilibili\.com|b23\.tv/i.test(q))) {
+    return { id: bv[0], platform: "bilibili" };
+  }
+
+  // NetEase song URL.
+  if (/music\.163\.com/i.test(q)) {
+    const m = /[?&#/]id=(\d+)/.exec(q) ?? /\/song\/(\d+)/.exec(q);
+    if (m) return { id: m[1], platform: "netease" };
+  }
+
+  // QQ song URL.
+  if (/y\.qq\.com/i.test(q)) {
+    const m = /songDetail\/([0-9A-Za-z]+)/.exec(q) ?? /[?&]songmid=([0-9A-Za-z]+)/i.exec(q);
+    if (m) return { id: m[1], platform: "qq" };
+  }
+
+  return null;
+}
+
+/**
+ * Detect a "#N" selection token (1-based) referencing the previous !search.
+ * Returns the positive integer, or `null` when the query isn't a selection.
+ */
+export function parseSelectionIndex(raw: string): number | null {
+  const m = /^#\s*(\d+)$/.exec((raw ?? "").trim());
+  if (!m) return null;
+  const n = parseInt(m[1], 10);
+  return Number.isFinite(n) && n > 0 ? n : null;
+}