feat(workgroup): freeze the natural-value protocol before its sample exists

Whether a turn's execution is worth offering as one foldable unit is a
question about ordinary use, and the only honest way to answer it is to watch
ordinary use. Designing tasks to cover the interesting cases would put the
task designer's choices into the granularity being measured, which is the one
thing this cannot afford.

So the protocol is fixed now, while no sample exists to fit it to. The first
twenty eligible authored turns after the freeze timestamp, in the order they
happen — not the last twenty, not a chosen twenty, and not twenty-four because
the answer landed near a line. Eligible means the turn carries a named
authored turn, which only the current schema emits and a synthetic
continuation never gets, and that its trajectory reads complete, because a
prefix cannot answer a question about a whole turn.

A turn with no tool calls is in the sample. The question is how much of
ordinary use has something to fold, and dropping the quiet turns would answer
a different one and flatter the feature. There are no route quotas either: the
mix of one-model and two-model turns is reported as a description of what
happened, never selected for.

The tool refuses to say anything but the count until the sample is whole.
Reading a partial result and acting on it is the sequential decision the
freeze exists to prevent, so there is nothing to read.

The verdict is one gate with both halves required — most groups holding a
single call AND few turns having three or more to fold — and failing it is not
an automatic go: a partition can be coarse and still rare, or common and still
thin, and those are different products.

The fold this measures with is the one the semantics pass settled, moved out
of that test rather than copied: two implementations of a partition drift, and
the assay and the fixtures have to be reading the same rules. The gate itself
runs on samples built to sit on each side of it, because an instrument nobody
has seen produce a number is one nobody has checked.

.gitignore

@@ -19,6 +19,7 @@
 /signpath-contract
 /covergate
 /repolint
+/workgroupassay
 /write_heartbeat
 /catalog-detector