Merge the batch export, and settle the seven-branch merge

Resolves the last of the parallel work. Two conflicts worth recording,
because both were semantic rather than textual:

`render_for_export` gained a colour space on master while the batch branch
was rewriting the single-image export path around it. Kept both: the batch
request supersedes the synchronous path, and the space still has to be chosen
at render time because the conversion happens in the shader before the clip to
0..1. `render_open_frame` takes it as an argument rather than reaching for a
controller it does not hold.

The map-wait moved into `readback::await_mapping` on one branch while another
was editing the constant it used, so `READBACK_POLL_LIMIT` survived the merge
with no callers. Removed rather than left for clippy to find later.

1164 tests pass, clippy clean, fmt clean. Traceability 53.0% -> 54.3%.

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

core/dr-gpu/src/adjust.rs

@@ -32,18 +32,6 @@ use crate::{DemosaicedImage, GpuContext, GpuError};
 /// reads them.
 const RESERVED_FIELDS: usize = dr_pipeline::RESERVED_UNIFORM_FIELDS;
 
-/// How many non-blocking polls a readback gets before it is called failed.
-///
-/// A bound rather than a spin forever: if the device is lost the map callback
-/// never arrives, and an unbounded loop would hang the interface rather than
-/// surfacing the error. Set far above any plausible completion — the copy this
-/// waits on is milliseconds — so it is reached only when something is wrong.
-///
-/// Ungated along with [`AdjustPass::export_pixels`], the one readback that
-/// survives S1: an export reads pixels back in a shipping build, and a lost
-/// device mid-export must surface as an error rather than a hung interface.
-const READBACK_POLL_LIMIT: u32 = 100_000;
-
 /// Runs composed operation chains against demosaiced images.
 pub struct AdjustPass {
     ctx: GpuContext,