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/import.rs

@@ -48,6 +48,7 @@
 //! record afterwards is each file's digest (`dr_catalog::dedup`), which the
 //! scan cannot know because it never reads a whole file.
 
+use crate::executors::{self, Executor};
 use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::mpsc::{Receiver, Sender};
@@ -228,14 +229,12 @@ pub struct Outcome {
 /// does, and the caller holds it.
 pub fn spawn(request: Request, cancel: Cancel) -> Receiver<Message> {
     let (tx, rx) = std::sync::mpsc::channel();
-    std::thread::Builder::new()
-        .name("import".into())
-        .spawn(move || {
-            if let Err(e) = run(request, &cancel, &tx) {
-                let _ = tx.send(Message::Failed(e));
-            }
-        })
-        .expect("spawning the import worker");
+    executors::try_spawn(Executor::Io, "import", move || {
+        if let Err(e) = run(request, &cancel, &tx) {
+            let _ = tx.send(Message::Failed(e));
+        }
+    })
+    .expect("spawning the import worker");
     rx
 }