fix(hub): a track with no artist tag reaches the Music grid

The grid's unit is an album, so a track whose artist tag is empty was
drawn nowhere — while `empty` counted it and stayed false, so no message
appeared either. An untagged library rendered a toolbar over a blank
page, with every track one mode-switch away and nothing saying so.

It gets a card, the same shape the singleton folding already mints. No
cover is looked up for it, or for any album this file invented: the
release name is one the browser wrote, and the request cannot match.

music_untagged_probe.py renders the real grid and reads the page back.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/CLAUDE.md b/CLAUDE.md
index 9aac263..694f7b2 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -493,6 +493,24 @@ do. Read them before writing anything that touches the same mechanism.
   viewport from the layout one), which was plausible, cost a round trip, and
   was wrong; the screenshot showing no keyboard was already in hand.
 
+- **A view that draws one shape of unit, beside an emptiness test that counts
+  every shape.** The Music grid's unit is an album; a track whose artist tag is
+  empty belongs to none, so it was drawn nowhere — while `empty` counted it and
+  therefore stayed false. Neither "no music" nor anything else appeared: a
+  toolbar over a blank page, with every track one mode-switch away and nothing
+  saying so. Both halves read as correct alone, which is why no amount of
+  re-reading the file found it; what found it was rendering the component
+  against entries with no artist and looking (`music_untagged_probe.py`).
+  **Whenever a mode filters the units it draws, the emptiness test has to be
+  asked of that mode, not of the library** — or the two disagree and the
+  disagreement is silent. Fixed by giving the untagged pile a card of its own,
+  the same shape the singleton folding already minted, so the two agree by
+  construction rather than by a second condition someone has to maintain. That
+  card also stopped a cover lookup going out for a release name the browser
+  itself wrote: a third-party request, on the operator's connection, that
+  cannot match anything, seen in a live log naming a placeholder as both artist
+  and release
+
 - **Three headers decide whether a page may frame itself, and they must agree.**
   The same streamed download navigates a hidden iframe to `/_mbdl/<id>`.
   `frame-src` was reCAPTCHA's two origins with no `'self'`, `frame-ancestors`