feat: let a mood exclude tags, and widen the eighties one to the canon

The eighties mood selected on synth-specific tags only, on the grounds that the
bare `80s` tag drags in Def Leppard and Bon Jovi. Checking the canon against
live Last.fm pages shows what that costs. Eurythmics is tagged 80s, new wave,
pop, female vocalists, synth pop -- four of those five are tags no mood may use.
Frankie Goes to Hollywood is tagged 80s, new wave, pop, british, dance, and
survives only on new wave. An act tagged purely 80s, pop and dance, which is a
great deal of eighties pop, was missed outright.

Two spellings also matter more than the census suggested. Of Depeche Mode, Duran
Duran, Eurythmics and Frankie Goes to Hollywood, three carry "synth pop" with a
space and only one carries "synthpop" without. The census could not show this
because it only sees artists already in the library; the spelling that was kept
was the rarer one.

Add an `exclude` list to a mood. An excluded tag drops the artist outright
rather than docking their score, which is the only construction that expresses
"the eighties, but not the stadium rock" -- weighting cannot do it, because the
tag it would weight is the one both share. The synth acts do not carry hard rock
or hair metal, and that is the whole of the difference.

A mood may not both select on and exclude the same tag; that is rejected when
the definitions are read rather than silently producing nothing.

README.md

@@ -188,7 +188,7 @@ measured, rather than from a general taxonomy:
 | `hair-metal` | hair metal, glam metal, glam rock, arena rock, AOR — 1975-1994 |
 | `nu-metal` | nu metal, alternative metal, rapcore, industrial |
 | `classic-rock` | classic rock, prog, psychedelic, blues rock, 70s, 60s |
-| `80s-synths` | synthpop, electropop, new wave, post-punk — 1975-1992 |
+| `80s-synths` | 80s, new wave, synth pop, electropop, post-punk — 1975-1992, rock excluded |
 | `indie` | indie, indie rock, indie pop, britpop, singer-songwriter |
 
 Measuring first mattered. `synthwave`, `edm`, `big room` and `hardstyle` are
@@ -205,9 +205,31 @@ Three kinds of tag are never used, and there is a test enforcing it:
   their own name. `green day`, `paramore` and `queen` are single-artist
   playlists waiting to happen.
 
-`80s-synths` deliberately excludes the bare `80s` tag, which in this library
-sits on Def Leppard and Bon Jovi rather than on anything with a synthesiser in
-it. Those belong to `hair-metal`, which is why that mood now exists.
+### Exclusions
+
+A mood may also list `exclude`. An excluded tag drops the artist outright rather
+than docking their score, and it exists because `80s-synths` cannot be written
+any other way.
+
+`80s` is the eleventh most-played tag here, and it sits on Def Leppard and Bon
+Jovi exactly as heavily as on Eurythmics. Weighting cannot separate them,
+because the tag it would weight is the one they share. What does separate them
+is that the stadium rock also carries `hard rock` and `hair metal`, and the
+synth acts do not.
+
+Checked against live Last.fm pages, since the tag census only sees artists
+already in the library:
+
+| Artist | Tags |
+| --- | --- |
+| Eurythmics | `80s`, `new wave`, `pop`, `female vocalists`, `synth pop` |
+| Frankie Goes to Hollywood | `80s`, `new wave`, `pop`, `british`, `dance` |
+| Depeche Mode | `electronic`, `synthpop`, `new wave`, `80s`, `synth pop` |
+| Duran Duran | `new wave`, `80s`, `pop`, `synth pop`, `rock` |
+
+Four of Eurythmics' five tags are ones no mood may use. Three of the four
+artists spell it **`synth pop`** with a space; only one spells it `synthpop`.
+Guessing one spelling would have missed most of the canon.
 
 An artist qualifies when their tag weights inside a mood sum to at least 30 out
 of Last.fm's 0-100 scale. One low-weight tag is not a genre, it is somebody's