SPL Meter 0.4.2: the device picker can build the tool it needs

NA could not list a single audio interface. The module was installed from
the catalog, which ships source only because the capture binary is
gitignored, and the picker shells out to that binary with --list. So it
returned nothing, forever:

  [spl-meter] device list failed: spawn .../spl-capture ENOENT

The missing-binary build existed, but only inside the listener, and the
listener only starts once a device has been picked. No tool meant no device
list, and no device meant no listener to build the tool. Nothing was going
to break that on its own.

The build now lives at module scope and both callers use it: init() asks for
it on mount whether or not a device is configured, and /devices asks again
each time it is called, so opening the admin page is enough to start it. It
is still a plain spawn that never touches the event loop, and health() now
names the situation instead of leaving an empty dropdown unexplained.

Also removed two per-listener build flags left shadowing the module-scope
ones, which would have made the "is a build already running" test always
false and let a second build start under the first.

Tested the way it actually failed: a data directory holding only the six
source files, no binary. The build starts on mount and /devices answers with
the real device list moments later.

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

modules/spl-meter/module.json

@@ -1,7 +1,7 @@
 {
   "id": "spl-meter",
   "name": "SPL Meter",
-  "version": "0.4.1",
+  "version": "0.4.2",
   "proddash": ">=1.5.0",
   "description": "Calibrated sound level, Leq and RTA from an audio input on this machine",
   "permissions": [