webui: replace nautctl web server with a loadable plugin

Drop the web UI that was compiled into nautctl and serve the
torrent-ui front end (../torrent-ui/public) from a native plugin
(plugins/webui) loaded via `nautd --plugin`. The plugin talks to the
engine only through the host call_rpc ABI and adapts the daemon's RPC
surface to the qBittorrent-style contract the UI expects (snapshot/SSE,
torrent detail tabs, add/delete, cookie auth).

Also folds in the daemon refactor that owns per-torrent worker threads
and the swarm engine (naut_swarm) used by the plugin's data source.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/scripting.md

@@ -16,18 +16,19 @@ them, and the `naut` API table.
 
 ## 1. Loading a script
 
-Pass a script to the daemon with `--script`:
+Load a script through the control API:
 
 ```sh
-nautd --socket /tmp/nautd.sock --script ./my-rules.lua
+nautctl --socket /tmp/nautd.sock script ./my-rules.lua
 ```
 
-- **One script per daemon.** If `--script` is given more than once, the last
-  path wins.
-- The file is **loaded and executed once at startup**, on the main thread,
-  before the event worker starts. Use this top-level run to define your hook
-  functions (and any state they need). If the file fails to load or its
-  top-level code raises, the daemon refuses to start and prints the Lua error.
+- **One active script per daemon.** Loading another script replaces the current
+  one. `nautctl unscript` unloads it. `nautd --script PATH` is also available
+  for startup configuration.
+- The file is **loaded and executed once**, synchronously on the control thread,
+  before its event worker starts. Use this top-level run to define your hook
+  functions (and any state they need). A load or top-level error rejects the
+  request (or prevents startup when `--script` is used).
 - After startup the script is **event-driven**: the functions you defined are
   called as matching events occur.
 
@@ -161,9 +162,9 @@ Move one **completed** file of a torrent to `destination` (the headline
 **Return value:** none on success.
 
 **Asynchronous semantics — important.** `move_file` does **not** perform the
-move inline on the script thread. It enqueues a bounded command that the daemon
-**owner thread** executes shortly after (this preserves the engine's
-shared-nothing threading: the script thread never touches storage directly).
+move inline on the script thread. It enqueues a bounded command that the
+torrent's **owner thread** executes shortly after (the script thread never
+touches storage directly).
 So a successful call means *"the move was accepted"*, not *"the file has moved."*
 The actual relocate (a `rename`, or copy+unlink across filesystems) runs later;
 its success or failure is reported in the **daemon log** and reflected in the
@@ -189,19 +190,15 @@ function on_file_complete(event)
 end
 ```
 
-**Prerequisite — register the torrent's storage first.** `move_file` resolves
-`torrent_id` through the daemon's torrent registry (`naut_session`). Register a
-torrent's storage with the `add_torrent` RPC before any move command can take
-effect:
+**Prerequisite — the torrent must be loaded.** Add the torrent to the daemon;
+the same worker that downloads it owns and executes its move commands:
 
 ```sh
-nautctl --socket /tmp/nautd.sock add_torrent \
-  '{"torrent_id":42,"torrent":"file.torrent","root":"/downloads/42"}'
+nautctl --socket /tmp/nautd.sock add file.torrent /downloads/42
 ```
 
-If `torrent_id` is unknown when the move drains, the relocate fails with
-"not found" in the daemon log (the Lua call itself still succeeded, because it
-only *queued* the command).
+If `torrent_id` is unknown or is being removed, `naut.move_file` raises a
+`move_file failed` error in the hook.
 
 ---
 
@@ -283,7 +280,7 @@ end
 Run it:
 
 ```sh
-nautd --socket /tmp/nautd.sock --script ./archive-on-finish.lua &
-nautctl --socket /tmp/nautd.sock add_torrent \
-  '{"torrent_id":1,"torrent":"big.torrent","root":"/downloads/1"}'
+nautd --socket /tmp/nautd.sock &
+nautctl --socket /tmp/nautd.sock script ./archive-on-finish.lua
+nautctl --socket /tmp/nautd.sock add big.torrent /downloads/1
 ```