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/library/xmp.rs

@@ -1,6 +1,7 @@
 //! Pushing local judgements out to XMP sidecars, and reloading sidecars a
 //! person chose to trust by hand.
 
+use crate::executors::{self, Executor};
 use dr_catalog::Catalog;
 use dr_sync::{Connection, RemoteBackend, RemoteId, RemotePath};
 use std::path::PathBuf;
@@ -32,7 +33,7 @@ pub struct XmpWrite {
 /// name. Reported once at the end, as the judgement writes are.
 pub fn spawn_xmp_writes(conn: Connection, writes: Vec<XmpWrite>) -> Receiver<XmpMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "xmp-write", move || {
         let mut written = 0usize;
         let mut failed = 0usize;
         let mut last_error = None;
@@ -140,7 +141,7 @@ pub fn spawn_xmp_reload(
     paths: Vec<String>,
 ) -> Receiver<XmpMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Network, "xmp-reload", move || {
         let mut written = 0usize;
         let mut failed = 0usize;
         let mut last_error = None;