frame-cost todo: option 3 is done, and what it did not fix

plan/todo/conducted-show-frame-cost.md

@@ -12,12 +12,15 @@ deployment.
 Two expensive scenes therefore land on the same frame at every seam.
 Worst crossfade per show, 6,660 lights (4A-37), this box:
 
-| show | main | after the arcs work |
-|---|---|---|
-| `koln` | 46.8 ms | 40.0 ms |
-| `apollo` | 32.8 ms | 31.4 ms |
-| `promises` | 53.2 ms | 51.9 ms |
-| `nocturne` | ~42–47 ms | unchanged (not edited) |
+| show | before either change | now (both merged) | worst seam is |
+|---|---|---|---|
+| `koln` | 31.8 ms | **31.9 ms** | `coming-to-rest` (NoiseGlow + Motif) |
+| `apollo` | 34.5 ms | **32.9 ms** | `matta` (AuroraVeils) |
+| `promises` | 54.1 ms | **50.2 ms** | `answer` (AuroraVeils into AuroraVeils) |
+| `nocturne` | 42–46 ms | **41.7 ms** | `deep-sea` (NoiseGlow + Starfield) |
+
+Repeat runs vary by a few ms; the *ranking* of seams is stable, the
+individual figures less so. Governor `performance`, 6,660 lights.
 
 A 30 fps frame is 33.3 ms. Steady-state scenes are fine — the same
 shows sit at 10–16 ms median — so this is a seam-only stall lasting as
@@ -32,11 +35,31 @@ octaves).
 
 ## What has already been done
 
-Nothing structural. Two local mitigations in `koln`, both cheap and
-both visible in the table above: the ballad's candle `count` is 52
-rather than 72 (cost is linear in `count`, and its three scenes are the
-one place in the repertoire where two `Candles` render at once), and
-the house-lights floor under Part IIc's stars runs at `octaves=2`.
+**Option 3 is done** (PR #43). `Candles` no longer evaluates its whole
+`n_lights x count` matrix: the light–flame pairs that carry any light
+are found once, cached on a content fingerprint, and the render walks a
+flat pair list. `Candles()` went 38.0 -> 3.7 ms, and nocturne's two
+candle seams went from 33–46 ms to under 23 — they are no longer
+anywhere near the worst thing in that show.
+
+It did not solve the problem, and was never going to: `promises` has no
+candles in it at all. What it did was remove one cause completely.
+
+Two local mitigations in `koln` also stand: the ballad's candle `count`
+is 52 rather than 72, and the house-lights floor under Part IIc's stars
+runs at `octaves=2`.
+
+## Where that leaves it
+
+`koln` (31.9 ms) and `apollo` (32.9 ms) now fit inside a 30 fps frame,
+`apollo` only barely. `promises` (50.2 ms) and `nocturne` (41.7 ms) do
+not, and both are bounded by `AuroraVeils` and `NoiseGlow` seams that no
+amount of candle work touches.
+
+The remaining options are 1, 2 and 4 below. **Option 1 is the only one
+that covers `promises`**, because the problem there is two expensive
+scenes of the same kind blended together, not one primitive being
+wasteful.
 
 ## Options, roughly in order of appeal
 
@@ -52,11 +75,10 @@ the house-lights floor under Part IIc's stars runs at `octaves=2`.
    cheap (both sides light) or expensive, and keep expensive-to-
    expensive seams short. Manual, but it is what the `koln` ballad now
    does by hand.
-3. **Make `Candles` cheaper.** The `(n, count)` pool matrix is the
-   single largest term in the repertoire. Candles beyond ~25° of a
-   light contribute nothing; a neighbour list computed once per
-   geometry (memoized on a content fingerprint, like `SmallPlanet`'s
-   statics) would turn it into a sparse gather.
+3. ~~**Make `Candles` cheaper.**~~ Done, PR #43 — see above. The
+   approach (neighbour list memoized on a content fingerprint, like
+   `SmallPlanet`'s statics) transfers directly to any other primitive
+   with per-entity pools, should one turn up.
 4. **Accept it and drop frames at seams.** The playout queue on the
    board absorbs a few late frames; a 10–30 s stretch at ~20 fps is
    not nothing, but it is a crossfade, and it is where the eye is
@@ -67,4 +89,6 @@ the house-lights floor under Part IIc's stars runs at `octaves=2`.
 None of this has been measured on the deployment box under load, only
 on the pattern render in isolation — no codec, no serial, no viewers.
 The real question is what `luminary play` does at a seam with six
-boards attached, and that has not been run.
+boards attached, and that has not been run. Until it has, even the two
+shows that now fit a 30 fps frame fit it *with nothing else running*,
+which is not the condition they will be played in.