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.rs

@@ -27,6 +27,7 @@
 //! sensor data stays on the CPU at a fifth of the size — and it is what
 //! makes a twelve-frame set fit.
 
+use crate::executors::{self, Executor};
 use std::collections::VecDeque;
 use std::path::{Path, PathBuf};
 use std::sync::mpsc::{Receiver, Sender};
@@ -727,7 +728,7 @@ fn run_inner(
     let fill_for_writer = fill_cam.is_some();
     let file =
         std::fs::File::create(&out_path).map_err(|e| format!("{}: {e}", out_path.display()))?;
-    let writer = std::thread::spawn(move || -> Result<(), String> {
+    let writer = executors::spawn(Executor::Io, "dng-write", move || -> Result<(), String> {
         let mut file = std::io::BufWriter::new(file);
         dr_export::write_linear_dng(
             &mut file,