Add a brightness-only sink: keep the grabber's colour, pulse with sound

Every existing HyperHDR route replaces whatever else is on the LEDs:
routes 1/2 hand HyperHDR's own audio effect a device to read, route 3
sends a synthetic spectrum image, and both take over via HyperHDR's
priority system. For a setup that already has a real colour source
(a screen grabber, a USB capture card) feeding an ambilight-style LED
run, none of that is what's wanted -- the colour should stay put and
only brightness should react.

Read HyperHDR's own source (sources/api/JSONRPC_schema/schema-adjustment.json)
rather than guess: "adjustment" is a post-processing command with a
scaleOutput parameter (0-2.0) that applies regardless of which
priority is currently active. Confirmed the wire format too --
sources/jsonserver/JsonClientConnection.cpp frames it as plain
newline-delimited JSON over TCP (default port 19444), nothing like the
length-prefixed Flatbuffers protocol the visualiser sink speaks, and
with no handshake or registration needed before the first write.

sink_hyperhdr_adjust.c sends only that: no image, no priority, so it
never competes with an existing grabber. Non-blocking connect with the
same poll()+SO_ERROR pattern net/hyperion.c already uses, reconnects
every 5s, rate-limited to 20 Hz (a HyperHDR command every audio block
would be pointless flooding), and resets scaleOutput to 1.0 on close
rather than leaving the LEDs stuck at whatever it last sent. Cross-
compiles clean under -Wall -Wextra on the real webOS toolchain.

Wired through the same path every other sink follows: registered in
sink.c/sink.h, defaults in config.c, fields in frontend/js/app.js
(SINK_FIELDS/SINK_HELP), mock.js and ui_smoke.js updated for the new
sink card. Bumped to 1.0.3.

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

test/ui_smoke.js

@@ -129,7 +129,7 @@ async function main() {
   eq('no sinks listed while stopped', $('sink-status').textContent.trim(), 'Not running.');
 
   console.log('panels');
-  eq('five sink cards', doc.querySelectorAll('.sink-card').length, 5);
+  eq('six sink cards', doc.querySelectorAll('.sink-card').length, 6);
   check('hyperhdr card is first and marked',
     doc.querySelector('.sink-card .badge').textContent === 'Recommended');
   check('hyperhdr host field exists', !!doc.querySelector('[data-path="hyperhdr.host"]'));