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/src/checks/phase11.js

@@ -26,6 +26,7 @@ import { Rng } from '../engine/rng.js';
 import { frameDistance, frameLuminance } from '../engine/hash.js';
 import { SHOT_SIZES, SHOT_SIZE_NAMES, describeFraming } from '../look/framing.js';
 import { reachFor, CURVE_NAMES } from '../look/Camera.js';
+import { subjectOf } from '../look/stack.js';
 
 /** Tracks at several tempos: shot length is measured in bars, so tempo is the axis. */
 let cached = null;
@@ -273,7 +274,7 @@ check(11, 'two tracks do not agree on what a section kind looks like', () => {
             directors.add(look.director);
             look.sections.forEach((section) => {
                 const set = seen.get(section.kind) || new Set();
-                (section.variants || [section.layers]).forEach((v) => set.add(v[0].module.family));
+                (section.variants || [section.layers]).forEach((v) => set.add(subjectOf(v).module.family));
                 seen.set(section.kind, set);
             });
         }
@@ -299,7 +300,7 @@ check(11, 'a track shows more of the library than it used to', () => {
         for (let s = 0; s < 4; s++) {
             const look = generateLook(t, { seed: (i * 2654435761 + s * 40503) >>> 0 });
             look.sections.forEach((section) =>
-                (section.variants || [section.layers]).forEach((v) => cast.add(v[0].module.name)));
+                (section.variants || [section.layers]).forEach((v) => cast.add(subjectOf(v).module.name)));
         }
     });
     const pool = scenes.filter(canBackground).length;
@@ -492,8 +493,11 @@ check(11, 'the slow axis is a journey rather than a cycle', () => {
 
     try {
         const cue = arc.cues[0];
-        const at = (time) => arc._paramsAt(cue, 0, time, t.at(Math.round(time * 60)));
-        const spec = arc._specFor(cue.sectionIndex, cue.variant, 0);
+        // The shot, not the bed under it — the slow axis is a claim about the
+        // scene the section is about.
+        const slot = arc._subjectSlot(cue);
+        const at = (time) => arc._paramsAt(cue, slot, time, t.at(Math.round(time * 60)));
+        const spec = arc._specFor(cue.sectionIndex, cue.variant, slot);
         const axis = arc._slowAxisFor(spec.module);
 
         const start = at(t.duration * 0.05);