Every shot stands on something, and the frame has two ends

A section used to be one scene, and two thirds of the library is composable —
sparse by design, elements ON something. Cast as backgrounds anyway, they left
9 of 40 sampled frames under 20% painted, the darkest at 0.3%: a minute and a
half of a few bright things on black, invisible to every gate because every
gate on the stack was a limit rather than a floor.

Every section now stands on a GROUND: a canvas that fills the frame, cast per
section kind so a shot cut changes the shot and not the world. When the shot
fills the frame itself it IS the ground — two canvases stacked is two pictures
fighting. Above that, a coverage BUDGET: director appetite times the section's
energy times where the story is, capped at two frames' worth of material.

The measured facts move into the repo. scenes/metadata.json is generated from
the gallery — coverage as a shot, coverage as a bed, variety, the structural
profile — tracked in git, stamped with a fingerprint of the scenes and the
metric definitions, and refreshed from gallery.html. `surface` is derived from
it rather than declared; nine scenes claimed `canvas` while painting under a
third of the frame, and declaring it is now a lint error. The generator weights
every layering choice by measured structural distance, because family labels
and the render disagree: two `geometric` scenes can be 0.31 apart and a `flow`
and an `organic` scene 0.04.

The gallery's 0.1 red line is gone. It was right when a section was one scene
and wrong now — nineteen scenes were failing a bar for being consistent, which
is a virtue in an ingredient.

Chasing the numbers turned up four real faults:

  * A scene that reads prev() cannot be a ground. It returns the whole
    composited frame including the layers above it, so a datamosh under a shot
    is eating it: the render stopped reproducing from a seek and two WebGL
    contexts diverged by 91/255 against a tolerance of 4.
  * Screen was the wrong operator for a shot over a bed. It lightens, so a
    median quarter of every frame clipped to paper and whole sections rendered
    100% white. Replaced by a lumakey — the shot's brightness is its alpha.
  * Feedback was an accumulator: a still image settled at 2.3x its own
    brightness. Fine over black, fatal over a filled ground. Normalised at 0.6,
    plus a highlight shoulder so the top rolls off instead of clipping.
  * useTrack never prewarmed, so a fresh Show's first frame differed from every
    later render of it — the export-breaking hazard Compositor.prime documents.

Blazing is a decision now, not a side effect: directors declare an appetite for
it, a section must be loud and late in the story to earn one, and quiet kinds
never do. The ceiling gate matches that — a hard cap per section, and no more
than a fifth of them hot at all.

Rendered across twelve videos, middle of every section:

    painted        51% mean, darkest 0.3%  ->  87% mean, darkest 43%
    clipped white  24% median, worst 100%  ->   1% mean, worst 30%
    separation     0.10                    ->  0.44

Seven scenes can ground a section — five geometric, two organic — so every
quiet section of every video stands on one of two beds. That is the library's
largest hole and it is scene work: there is no minimal or flow canvas that
fills half the frame without reading prev().

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flow-state/HOWTO-variety.md

@@ -28,16 +28,23 @@ checks.html?scene=X   the per-scene gate, including `consumes:` lines
 ```
 
 The gallery score is the mean structural distance between a scene's own six
-frames, on the same descriptor the variety harness uses. **Minimum bar: 0.1**
-(`MIN_VARIETY` in `checks/gallery.js`), drawn as a red line. Below it a scene is
-the same picture wherever it appears, and because it is cast into many songs
-that sameness leaks between them — the viewer recognises the shot rather than
-the song.
+frames, on the same descriptor the variety harness uses. High means the song
+changes this scene a lot; low means it looks like itself wherever it is cast.
 
-Most of the library does not clear it. That is deliberate: a bar set where the
-work already is measures nothing.
+**There is no bar.** There used to be one — 0.1, drawn across the gallery as a
+red line — and it was removed when composition arrived. A section is no longer
+one scene: it is a ground, a shot standing on it, and sometimes a pass over
+that, so what a viewer sees is a COMBINATION. A scene that is reliably itself is
+a perfectly good ingredient in one, and the old line failed it for being
+consistent.
 
-Some reference points, measured:
+What replaced it is a level up. The interesting quantity is how unalike the
+scenes in one stack are, and the generator now weights its choices by exactly
+that — see `structuralDistance` in `scenes/surface.js`. Your scene does not have
+to be varied on its own. It has to be unlike the things it will be stacked with,
+and that is measured for you.
+
+Some reference points, measured across six songs:
 
 ```
 0.324  Droste Feedback      varies a lot
@@ -47,6 +54,29 @@ Some reference points, measured:
 0.047  Moiré Grid           after a subject; 0.031 before
 ```
 
+### The measured metadata
+
+`src/scenes/metadata.json` is generated, tracked in git, and holds every
+measured fact about every scene: coverage, variety, per-block scores and the
+structural profile the distance above is computed from. Nothing in it is
+hand-written, and two decisions read it:
+
+* **`surface`** — canvas at 50% coverage or more, composable below. It used to
+  be declared per scene and it drifted; nine scenes claimed `canvas` while
+  painting under a third of the frame. Declaring `surface:` in a scene file is
+  now an error.
+* **which scenes may be a GROUND** — a canvas that does not call `prev()`, fills the frame
+  when sampled as a bed, and does not vanish under any of the six identities. Coverage is
+  measured twice, once as a shot and once as a bed, because a scene paints wildly different
+  amounts at different parameters: Metaballs is 69% as a drop's shot and 6% as an intro's
+  ground.
+
+Refresh it from **gallery.html → refresh metadata** after changing a shader or a
+metric. The file carries a fingerprint of the scenes, the shader contract, the
+identities and palettes, and the descriptor definitions; when that stops
+matching, phase 12 fails and tells you to re-measure. The write goes through a
+dev-only endpoint in `vite.config.js`, so it only works under `npm run dev`.
+
 ---
 
 ## What the descriptor can and cannot see