feat(producer): open the DE single-worker inversion to short comps under an element ceiling

31% of fleet renders (24h, v0.7.78+) are DE-eligible comps clamped to
parallel screenshot purely because they sit under the 900-frame inversion
floor — the median fleet render is ~250-600 frames, below every DE entry
threshold. This opens a 250-899 frame band, gated on composition size.

Measured, not assumed. A controlled sweep (fixed synthetic content,
{250,400,600,900}f, single-DE vs parallel-screenshot-W4, 3 reps, capture
mode verified per row, AC power, load-gated) showed single-DE winning
1.16-1.24x at every size — but only for content in constant motion. A
follow-up 2x2 found motion and DOM size pull in OPPOSITE directions, so
neither alone predicts the winner (ratio = ss4/de1, >1 means DE wins):

     24 movers /     0 nodes -> 1.05
    320 movers /     0 nodes -> 1.24
    320 movers /  7000 nodes -> 1.09
     24 movers /  7000 nodes -> 0.96
     24 movers / 20000 nodes -> 0.71
     24 movers / 40000 nodes -> 0.55

DE's wall-clock scales ~0.50ms/element against parallel screenshot's
~0.22ms — drawElement repaints the whole tree per frame while fan-out
amortizes it — so the downside is NOT bounded and a bare floor drop would
have handed a 1.8x regression to large comps. Since motion only ever helps
DE, an element ceiling calibrated at the lowest-motion case is safe at
every motion level; crossover there is ~3.9k, and the default sits at 2500.

The predicate is untouched; the call site picks the floor. Above the
ceiling, or at 900+ frames, behaviour is bit-identical to today — the
change can only add inversions in the new band, never remove one.

Instrumentation, since this ships at full exposure rather than cohorted:
`composition_element_count` on EVERY render (the fleet distribution of the
gate variable is unknown — without it we cannot tell whether 2500 opens the
band for most short comps or almost none, nor re-derive the threshold from
real content), and `de_short_band` = applied | skipped_elements, unset when
the frame count made the band irrelevant, so a fleet perf shift is
attributable to this change rather than to content mix.

Safety is unchanged and already proven on this path: per-frame PSNR
self-verify with screenshot fallback, exactly as the 900+ band has shipped
default-on. Knobs: HF_DE_SHORT_MIN_FRAMES, HF_DE_SHORT_MAX_ELEMENTS (0
disables the band).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packages/cli/src/telemetry/renderObservability.ts

@@ -42,6 +42,8 @@ export function renderObservabilityTelemetryPayload(
     captureMemoryExhaustionDetected: capture.memoryExhaustionDetected,
     captureDeWorkerInversion: capture.deWorkerInversion,
     captureDePreInversionWorkers: capture.dePreInversionWorkers,
+    captureCompositionElementCount: capture.compositionElementCount,
+    captureDeShortBand: capture.deShortBand,
     captureDeParallelRouter: capture.deParallelRouter,
     captureDeGpuRenderer: capture.deGpuRenderer,
     captureDePreRouterWorkers: capture.dePreRouterWorkers,