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/bursts.rs

@@ -36,6 +36,7 @@
 //! abandoned half way leaves the signatures it did compute — they are permanent
 //! and correct — and the previous grouping intact.
 
+use crate::executors::{self, Executor};
 use std::cell::{Cell, RefCell};
 use std::collections::HashMap;
 use std::path::PathBuf;
@@ -82,7 +83,7 @@ pub enum BurstMessage {
 pub fn spawn_grouping(catalog_path: PathBuf, thumbs_dir: PathBuf) -> Receiver<BurstMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Decode, "bursts", move || {
         let catalog = match Catalog::open(&catalog_path) {
             Ok(c) => c,
             Err(e) => {