Distil a slot head and record it beside the other two (V-661)

BIO tags had no Maven-domain corpus, which was true of found corpora and
false of made ones. A GBNF closed over Maven's five slots plus a
substring check gives 2178 spans out of gemma-4-12b at no second call.

Three heads over one forward pass: intent 92.8%, destination 82.8%, slot
span F1 72.4% over three seeds. The slot head is free.

Epoch selection reads the intent dev slice, so it stops the slot head
about 4 points early. Recorded rather than fixed.

CLAUDE.md

@@ -258,9 +258,18 @@ Recall is 15/15 and world is 5/5. Intent is 93.6% mean over three seeds. That is
 clarify class, so the head's fixture is the 88 cases carrying an intent.
 
 **Mood is cut, not deferred.** The enum describes her own reply state, not the
-speaker's emotion, and no dataset maps onto it. **BIO slot tags have no
-Maven-domain corpus**, so they stay in the MASSIVE body from step 2. Both are
-label problems and neither is a GPU problem: the run is under four minutes.
+speaker's emotion, and no dataset maps onto it.
+
+**A third head landed the same day** (`docs/evals/2026-08-08-slot-head-three-head.md`).
+BIO slot tags had no Maven-domain corpus, which was true of found corpora and
+false of made ones. `label_slots.py` distils spans out of gemma-4-12b under a
+GBNF closed over Maven's own five slots. A span survives only when it is a
+literal substring of the utterance, so the agreement filter costs no second
+call. 2178 spans over 1702 rows, 37 dropped, nothing unparsed. Three heads score
+intent **92.8%**, destination **82.8%** and slot span F1 **72.4%** over three
+seeds. The slot head is free: both other numbers move less than their own seed
+spread. Epoch selection reads the intent dev slice alone. Slot F1 is still
+climbing when it stops, which costs about 4 points.
 
 The MASSIVE warm-start of step 2 is worth nothing here. Stock e5-small ties it on
 intent and leads by a third of a case on destination. Nothing argues for keeping