Stand each person in the grouping pass by at most 100 references

Every face the user has ruled on entered the pass as an anchor, and the
scan is exhaustive by design (`dr_face::neighbours`), so a person with
750 confirmed faces cost 750 comparisons against every other face in
the library — and the cost of a library grew with how well it was
named. Most of those comparisons said nothing new: thirty frames from
one afternoon are one point of view, not thirty, and a face that
matches one of them matches the rest.

Each person now enters through at most 100 of their anchored faces
(`dr_face::references`). Eligible are those whose raw embedding is at
least 15 long — one above the gallery floor, since a reference speaks
for someone rather than merely being admitted — with an unmeasured
length admitted as it is everywhere else. From those, the set spanning
the greatest volume is chosen greedily: the longest vector first, then
at each step the face with the largest component orthogonal to the
chosen so far. That is pivoted Gram–Schmidt, and the product of the
residuals it picks is the Gram determinant, so the greedy step is the
exact greedy on the objective. A near-duplicate of a chosen face has
no residual and is passed over; the one profile shot among two hundred
frontal frames is taken early; faces inside the span of the chosen add
no volume and are not taken to fill the cap.

The faces not chosen keep their confirmations and are not touched by
the pass — they stay in the anchor map, so it never releases them —
they are simply not compared. A person none of whose faces is long
enough is still stood for, by their longest, rather than losing their
anchor and having their next face filed as a stranger. Under the cap
nothing changes: every eligible face stands, and the short ones stay
in as the probes they were.

At the reference library's 3,851 confirmations the scan shrinks by
about a fifth; at 15,000 it is a fifth of what it was.

docs/faces.md

@@ -821,6 +821,16 @@ here, and it is the phone and tablet story that should decide whether it gets bu
   already has, the raw vector written over the old one and its id, box and identity untouched
   (`faces::record_updates`). No detector runs and no suggestion is lost — the cost is the
   original fetched once more, since the length exists only at the moment of embedding.
+- **A person is stood for by their references.** Every face the user has ruled on is an anchor,
+  and the scan is exhaustive, so a person with 750 confirmations would cost 750 comparisons
+  against every other face — and the cost of a library would grow with how well it was named.
+  Instead each person enters through at most `MAX_REFERENCES` (100) of their anchored faces,
+  chosen by `dr_face::references`: those whose raw embedding is at least `MIN_REFERENCE_QUALITY`
+  (15) long, and among them the set spanning the greatest volume — greedy max-determinant, the
+  longest vector first and then, at each step, the face with the largest component orthogonal to
+  the chosen so far. Thirty frames from one afternoon contribute one reference; the single profile
+  shot is taken early. The faces not chosen keep their confirmations and are not touched by the
+  pass; they are simply not compared.
 
 **The algorithm.** Constrained average-link agglomeration over the probability graph, merging while
 the average pairwise probability exceeds **0.9** and no cannot-link is violated. Average-link rather