Frame every model in the tour, and measure that it does

The showcase is the one deliverable a still render cannot check, and until now
the only way to check it was to watch it -- which is how a model half out of
frame gets shipped. `__probe.framing()` projects the current model's bounding
box into the frame and reports the worst corner in normalised device
coordinates: 0 is comfortably inside, 1 is exactly on the edge. The whole 60
seconds can then be swept and the bad moments found by number.

Swept, it found five models leaving the frame. Four of them were during the dip
between beats, where the veil is 92% opaque and the point is that you cannot see
the swap -- so the honest figure is two frames out of 1060, the broom and the
ladder, each for a twentieth of a second. Not much. Also not nothing, when the
whole point of the tour is that someone records it once and posts it.

The first fix was to widen the framing margin from 1.55 to 1.62. It worked and
it was the wrong fix: it took a slice of the frame off all thirty-five models to
buy back two frames from one, and mean fill fell to 0.515 -- every model sitting
in half a screen of empty space.

The right fix is in the chase. The camera's distance follows the model's size by
exponential approach at a single rate, and the two directions are not the same
mistake: falling behind a model that is GROWING crops it, and falling behind one
that is shrinking only leaves it small for a moment, which nobody notices. The
broom was the last offender because its bundle and its handle both sweep. So the
camera now retreats at 18 and closes at 7, and with that in place the margin
came back in past where it started, to 1.38.

Measured over the visible frames of a full 60 s run: no frame crops, the closest
any model comes to the edge is 0.81, and mean fill is 0.605 against 0.515 for
the wide-margin version and 0.55 for the original. Models are bigger in frame
than before AND nothing is cut.

Also here: the tour verified end to end -- 1201 frames, 60.05 s, 35 models, no
repeats, chest to oak -- and two stale numbers in the README. It claimed ~800
checks (1,044) and did its showcase arithmetic for 27 models, which is the kind
of sentence that goes quietly wrong every time a model is added, so it now says
what the number does when the kit grows.

One correction to my own measurements, twice over. `showcase.update` clamps its
delta to 0.05 internally, which `frameStep` does not, so stepping 0.2 advances
0.05 -- my first two readings of the tour's length were wrong by 4x and 10x and
disagreed with each other, which is what gave it away. And the run that
reported 34 models in 57.8 s was racing a page reload, not a dropped model.

my-registry/models/oak-tree/model.ts

@@ -113,7 +113,7 @@ export function createModel(overrides: Partial<OakTreeConfig> = {}) {
        * photograph by finding the run of trunk pixels at that height, and the
        * profile they make is the whole character of the bole: a violent flare
        * in the bottom twentieth, then a shaft that hardly tapers for a quarter
-       * of the tree, then a steady run out to nothing at 78%.
+       * of the tree, then a steady run out to nothing at 74%.
        */
       const TAPER: ReadonlyArray<readonly [number, number]> = [
         [0.00, 1.95],