R7: extract the M3U regeneration into a shared sourced helper

run-playlist.sh and import-track.sh had the same beets->#EXTM3U block, but
import-track.sh wrote the file non-atomically. Move it to pipeline/lib/m3u.sh
as regen_m3u <name> <out>, sourced by both. The helper always writes atomically
(temp + rename), so a reader like Navidrome never sees a partial file and a
stray wrong-owner leftover .m3u8 is replaced without a chown. Returns 0 always
(empty result is a warning), so it's safe as a standalone call under set -e.

Verified: tests/test_m3u.py (atomic write with tracks, no file when empty; 44
tests green) plus a live regen against the real library (techno -> 229 tracks,
no temp leftover).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pipeline/lib/m3u.sh

@@ -0,0 +1,37 @@
+#!/bin/bash
+# Sourced helper: regenerate one playlist's .m3u8 from the beets library.
+#
+#   regen_m3u <playlist_name> <out_path>
+#
+# Writes atomically (temp file in the same dir, then rename over the target):
+# rename(2) is atomic for a reader like Navidrome, and needs write access only
+# on the directory, so it also replaces a stray wrong-owner leftover .m3u8 (e.g.
+# from a pre-container run) without a host-side chown.
+#
+# Relies on the caller's log() function and $LOG. Always returns 0 (an empty
+# result is a warning, not a failure), so it is safe to call as a standalone
+# statement under set -euo pipefail.
+regen_m3u() {
+  local playlist_name="$1"
+  local out="$2"
+  mkdir -p "$(dirname "$out")"
+
+  local beet_output
+  # || true: beet ls exits non-zero on no match, which would trip set -e.
+  beet_output=$(beet ls -f '$path' "grouping:${playlist_name}" 2>>"$LOG" || true)
+  local track_count=0
+  [[ -n "$beet_output" ]] && track_count=$(printf '%s\n' "$beet_output" | wc -l)
+
+  if [[ "$track_count" -gt 0 ]]; then
+    local tmp="${out}.tmp.$$"
+    {
+      echo "#EXTM3U"
+      echo "$beet_output"
+    } > "$tmp"
+    mv "$tmp" "$out"
+    log "M3U updated with $track_count tracks"
+  else
+    log "WARNING: beet list returned no tracks for grouping:${playlist_name} — not updating M3U"
+  fi
+  return 0
+}