Say where the positives really come from, not where they were going to

`calibrate.rs` claimed its positive pairs came from user confirmations "then
burst siblings, since FR-CULL-5 already groups bursts", and repeated it beside
the pair floor: "the positives are bootstrapped from bursts and a handful of
early confirmations". Neither is true and neither ever has been. Nothing in
the workspace pushes a burst pair into a `Pairs`; nothing pushes any pair at
all outside this file's own tests. The sentence was written while both halves
of docs/faces.md §8.1 were being planned together, describing a source that
was going to exist, and it has read since as a description of what the code
does.

The distinction matters more here than in most comments, because this file is
the one place in the subsystem allowed to say what a similarity *means*. A
reader who believes the fit is drawing on bursts believes a young library is
gathering positives on its own, which is precisely the opposite of the state
FR-CULL-9 legislates for — a library with no fit, no valid calibration, and a
reference curve it must not present as a measurement of itself. The floor of
200 positive pairs looks arbitrary under the wrong story and obvious under the
right one: confirmations arrive one at a time, from a person.

So the comment now says what is here — a positive is a pair confirmed onto one
person, and there is no second source — and keeps the burst idea where it
belongs, as §8.1's proposal, with the two reasons it is not in the code: this
crate is handed cosines and cannot see a catalog, and the purity of a burst
pair is a thing to measure before it is a thing to trust.

No behaviour changes; the arithmetic is untouched.

core/dr-face/src/calibrate.rs

@@ -21,11 +21,20 @@
 //! fails. The exceptions (mirrors, photographs of photographs, collages) are
 //! rare enough to be noise at this scale.
 //!
-//! **Positives have to be earned.** In order of trustworthiness: pairs the user
+//! **Positives have to be earned.** A positive is a pair of faces the user has
-//! has confirmed onto one person; then burst siblings, since FR-CULL-5 already
+//! confirmed onto one person (FR-CULL-10), and there is no second source: the
-//! groups bursts and two faces in adjacent frames are near-certainly the same
+//! only labelling this subsystem has is the labelling somebody did by hand.
-//! person. Nothing else — bootstrapping positives from high cosine is circular,
+//! Bootstrapping positives from a high cosine is circular — it fits the
-//! fitting the calibration to the belief it was supposed to test.
+//! calibration to the belief it was supposed to test — and that is the whole
+//! of the alternative.
+//!
+//! docs/faces.md §8.1 names one more that would cost no labelling at all: two
+//! faces in adjacent frames of one burst are near-certainly the same person,
+//! and FR-CULL-5's grouping is sitting there. Nothing draws on it. This crate
+//! cannot see a catalog, let alone the bursts in one — it is handed cosines by
+//! whoever assembled the pair — and no caller does that assembly on its behalf
+//! yet. Until one does, and until the purity of a burst pair is *measured*
+//! rather than assumed, the positives are the confirmations and nothing else.
 //!
 //! Which is why a fresh library has **no valid calibration** — no fit of its
 //! own — and says so. It is not left without a curve: it uses the reference
@@ -44,9 +53,10 @@ const BINS: usize = 200;
 /// Far stricter than the reference implementation's floor of two positives and
 /// one negative. That floor is reasonable there: its pairs come from a curated
 /// gallery of labelled reference portraits, where a positive pair is
-/// trustworthy by construction. Here the positives are bootstrapped from bursts
+/// trustworthy by construction. Here every positive is a pair somebody
-/// and a handful of early confirmations, and the whole risk is fitting
+/// confirmed while working through a young library's suggestions — a handful,
-/// confidently to too few of them.
+/// arriving slowly — and the whole risk is fitting confidently to too few of
+/// them.
 pub const MIN_POSITIVE_PAIRS: u64 = 200;
 pub const MIN_NEGATIVE_PAIRS: u64 = 2_000;