Timers: built-in LTC listener, configured in the admin page

The LTC listener now lives inside the module: it spawns the native
ltc-capture helper (moved into the module folder, compiled on first use,
exact-name device matching) for the admin-selected input device, extracts
the configured channel, and decodes SMPTE LTC in-process (ltc.js — the
engine, shared with tools/ltc-reader.js, which remains the remote-machine
option posting to /ltc).

Admin config: an "LTC listener" switch, an audio-device select populated
live from the new GET /ltc/devices route, and a 1-based channel number.
The switch off means LTC is gone entirely: no LTC entry in the Add-tile
picker, no LTC row in the Timers menu or All-timers grid, no stage/API
probes, and POST /ltc answers 409. Source precedence with it on:
listener-with-data, fresh remote reader, listener's own no-signal verdict,
then the legacy API/stage chain.

Shell (admin page): `select` fields learn `optionsRoute` — options fetched
from a module route at render, saved value kept selectable when the route
or device is gone. Documented in MODULE-GUIDE; backward compatible.
Coordinate with the shell lane before its next admin.js change.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/MODULE-GUIDE.md

@@ -67,10 +67,19 @@ Both schemas map field names to specs:
 { "type": "string" | "number" | "boolean" | "select" | "password" | "endpoint",
   "label": "Shown next to the field",
   "default": <value>,
-  "options": [ { "value": "a", "label": "A" }, … ]   // "select" only
+  "options": [ { "value": "a", "label": "A" }, … ],  // "select" only
+  "optionsRoute": "/devices"                          // "select" only, admin config only
 }
 ```
 
+A `select` whose choices are only knowable at runtime (audio devices, serial
+ports, discovered sources) sets `optionsRoute` instead of — or as a fallback
+refresh of — static `options`: the admin page GETs
+`/api/modules/<id><optionsRoute>` and expects `{ "options": [ { "value",
+"label" }, … ] }` from one of the module's own server routes. The saved value
+stays selectable even when the route is down or the option has vanished. (See
+`propresenter-timers`' "LTC audio device" for a worked example.)
+
 `password` fields are admin-config only in practice: their values are stored
 in `config/modules.json`, handed to your *server* entry, and **never sent to
 browsers** — not in `/api/modules`, and never echoed back into the admin