fix(media): a tag-scan placeholder is not a book identity

The first real plan filed 1,106 chapters from 14 unrelated top-level folders
onto one shelf, because all 1,106 carry the album tag [Unknown Album] and the
same artist tag. Plex's album title is the authority for the shelf only when it
names a book; a placeholder names nothing, and unrelated books share it. Those
chapters now park with the path they have today, timestamp-glued variants
included; Various Artists stays a legitimate multi-cast narrator.

tests/media_audiobooks.bats

@@ -126,6 +126,23 @@ setup() {
     at "$G/NoExt/part01" 9000
     row "$G/NoExt/part01" "Archivist" "Tapes" 1971 1 "Tape One"
 
+    # Tag-scan placeholders. Two unrelated books whose album rows both read
+    # `[Unknown Album]`: the name is shared, so filing on it would merge books
+    # that have nothing to do with each other. A rip timestamp can be glued to
+    # the placeholder, and a multi-cast read says `Various Artists` — which IS a
+    # narrator.
+    H="$AB/Packs"
+    at "$H/Stross/Ship/c1.mp3" 9000
+    at "$H/Leckie/State/c1.mp3" 9000
+    at "$H/Rip/tape.mp3" 9000
+    at "$H/NoWho/Real Book.mp3" 9000
+    at "$H/Multicast/Genesis.mp3" 9000
+    row "$H/Stross/Ship/c1.mp3" "Various Artists" "[Unknown Album]" "" 1 "Part 1"
+    row "$H/Leckie/State/c1.mp3" "Various Artists" "[Unknown Album]" "" 1 "Part 1"
+    row "$H/Rip/tape.mp3" "Ann Reader" "Unknown album (03/03/2015 21:39:55)" "" 1 "One"
+    row "$H/NoWho/Real Book.mp3" "[Unknown Artist]" "A Named Book" 2001 1 "One"
+    row "$H/Multicast/Genesis.mp3" "Various Artists" "The Bible" "" 1 "Genesis"
+
     # The enum TSV is what scripts/media-census.py enumerate writes: bytes, ext,
     # path, no header.
     python3 - "$AB" "$ENUM" <<'PY'
@@ -319,6 +336,28 @@ PY
     [ "$(kind "$G/blank narrator.mp3")" = "park" ]
 }
 
+@test "books that share a placeholder album name never share a shelf" {
+    plan
+    [ "$(dst "$H/Stross/Ship/c1.mp3")" = "$PARK/Packs/Stross/Ship/c1.mp3" ]
+    [ "$(dst "$H/Leckie/State/c1.mp3")" = "$PARK/Packs/Leckie/State/c1.mp3" ]
+}
+
+@test "a placeholder with a rip timestamp glued to it is still a placeholder" {
+    plan
+    [ "$(kind "$H/Rip/tape.mp3")" = "park" ]
+}
+
+@test "a narrator placeholder parks even when the book is named" {
+    plan
+    [ "$(kind "$H/NoWho/Real Book.mp3")" = "park" ]
+}
+
+@test "Various Artists is a multi-cast narrator and files its book" {
+    plan
+    [ "$(dst "$H/Multicast/Genesis.mp3")" = \
+        "$AB/Various Artists/The Bible/Genesis.mp3" ]
+}
+
 @test "the row, not the extension, decides what a chapter is" {
     plan
     [ "$(kind "$G/NoExt/part01")" = "chapter" ]