#3335: document EFFECTCRAFT_RQ_RENDERER and the Render Queue's per-job compositor choice

architecture.md and web.md still said every job renders on the project's
renderer (#414); since #3335 a GPU job times both compositors on its first
frames and keeps the faster.

docs/architecture.md

@@ -340,10 +340,13 @@ whichever compositor measured faster for it (`render::auto::AutoPick`, kept by t
 per comp, output scale and path (readback or viewer display) it averages CPU and GPU frame times
 (warm-up on both sides, the fastest warm-up frame starting the average, single stalls capped),
 picks the GPU unless the CPU is clearly faster, and re-measures the other side every 48 frames.
-Render Queue jobs don't use Auto: every frame of a job renders on the project's renderer (the
-GPU compositor with GPU Acceleration), so runs of a render don't differ by timing.
 Light comps (a few small layers, which the CPU composites only within their bounds) then stay on
-the CPU instead of paying for full-frame passes and a readback. A GPU frame walks the comp like
+the CPU instead of paying for full-frame passes and a readback. Render Queue jobs don't use
+Auto per frame: a job renders every frame on one compositor, never a mix (#3335). A GPU
+Acceleration job with an accelerator times both on its first frames
+(`export::pipeline::Cx::probe`, jobs of 12 frames or more) and keeps the faster, the CPU only
+when clearly faster; `EFFECTCRAFT_RQ_RENDERER=gpu|cpu` fixes the choice, and readback-deferred
+accelerators (the web) render on the project's renderer. A GPU frame walks the comp like
 `draw_comp`: cached layer buffers are uploaded once per buffer, then transformed with the CPU's
 sampling (nearest, bilinear, Catmull-Rom bicubic, the same minification pre-filter), motion-blur
 sub-samples are accumulated, and track mattes, Preserve Transparency, layer style passes,

docs/preferences.md

@@ -105,7 +105,11 @@ Commands (CLI, MCP, control channel):
   flight of every Render Queue render and export, `effectcraft-cli render` included
   ([#1615](https://github.com/storytold/effectcraft/issues/1615)); previews don't read it. One
   exception: while a job on GPU Acceleration times both compositors on its first frames, it
-  renders them under the cap but keeps them all (up to 12) until it has chosen.
+  renders them under the cap but keeps them all (up to 12) until it has chosen. That timing
+  picks the compositor that a job of 12 frames or more renders every frame on (the render log
+  names it and why). `EFFECTCRAFT_RQ_RENDERER` set to `gpu` or `cpu` skips it and renders on
+  that compositor; `auto` (the default) measures. A queue item's later output modules reuse its
+  first one's choice, and Software Rendering projects always render on the CPU.
 - `threeD.defaultRenderer`: Default 3D Renderer
 - `labels.N.name` / `labels.N.color`: the 16 label names and colours, used by the Label menu,
   the timeline, project panel, render queue and viewer handles

docs/web.md

@@ -173,9 +173,9 @@ the same futures with `passes::block_on`; they never wait there).
 
 - **Render Queue**: `effectcraft_export::export_async` renders each frame in passes (frames
   with deferred readbacks render one by one instead of in parallel batches) and encodes as
-  before (`Exporter::export_async`). As on the desktop, a job renders every frame on the
-  project's renderer (the worker's GPU with GPU Acceleration), not on whichever
-  compositor measured faster, so runs don't differ by timing.
+  before (`Exporter::export_async`). Unlike the desktop, which times both compositors on a
+  job's first frames and keeps the faster, a job renders every frame on the project's renderer
+  (the worker's GPU with GPU Acceleration), so runs don't differ by timing.
 - **Analyses** (Warp Stabilizer, 3D Camera Tracker, Track Motion, mask and face tracking, Roto
   Brush freeze and propagation): their work functions are `async`; started with `wait` inside
   a job they are queued and awaited (`offload::run_or_queue`), and each input frame renders in