Start every worker thread through the executors module

Thirty-nine spawn sites in dr-ui, and one in the Android entry point,
called std::thread::spawn or a Builder of their own, and most of the
threads they started were <unnamed> in a panic message or a profiler.
Each now calls executors::spawn with its executor and a role, so the thread is
named <executor>:<role> — net:sync, decode:thumbs, io:catalog-open —
and knows which executor it is on. The three that already set a name
(automation, import, prefetch) keep their name as the role.

Behaviour is unchanged: each job still gets a thread of its own when it
starts, and spawn panics where std::thread::spawn did.

The module's documentation now says how a job is assigned: by what it
spends its time on, so a sweep that fetches bytes and then decodes them
is Decode, and a sidecar write that touches the catalog is Network.

Left as they were: the segmentation and refine workers in masks_ui.rs,
which another change is reworking, and test-only threads.

ui/dr-ui/src/merge_ui.rs

@@ -11,6 +11,7 @@
 //! staged, the outbox drains and the library rescans, and the composite
 //! appears in the grid beside its sources.
 
+use crate::executors::{self, Executor};
 use std::cell::{Cell, RefCell};
 use std::rc::Rc;
 use std::sync::mpsc::{Receiver, Sender};
@@ -446,7 +447,7 @@ fn start<Fetch>(
     {
         let cancel = cancel.clone();
         let tx = tx.clone();
-        std::thread::spawn(move || {
+        executors::spawn(Executor::GpuSubmit, "merge", move || {
             let Some(frames) = fetch(&tx, &cancel) else {
                 return;
             };