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/faces.rs

@@ -23,6 +23,7 @@
 //! on every one. It therefore runs debounced, when detection has been idle and
 //! the face count has moved materially (catalog.md §10.2).
 
+use crate::executors::{self, Executor};
 use std::path::PathBuf;
 use std::sync::mpsc::{Receiver, Sender};
 
@@ -816,7 +817,7 @@ pub fn spawn_store_face_sweep(
 ) -> Receiver<FaceSweepMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Decode, "faces", move || {
         let finish_empty = |tx: &Sender<FaceSweepMessage>| {
             let _ = tx.send(FaceSweepMessage::Finished {
                 images: 0,
@@ -1348,7 +1349,7 @@ pub fn spawn_grouping_preview(
 ) -> Receiver<PreviewMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Decode, "grouping", move || {
         let msg = match Catalog::open(&catalog_path) {
             Ok(catalog) => match preview_grouping(&catalog, &model_id, &grouping) {
                 Ok(p) => PreviewMessage::Ready(p),
@@ -1397,7 +1398,7 @@ pub fn spawn_recluster(
 ) -> Receiver<ReclusterMessage> {
     let (tx, rx) = std::sync::mpsc::channel();
 
-    std::thread::spawn(move || {
+    executors::spawn(Executor::Decode, "recluster", move || {
         let catalog = match Catalog::open(&catalog_path) {
             Ok(c) => c,
             Err(e) => {