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>

README.md

@@ -105,6 +105,7 @@ Each can run at the same time as the others.
 | --- | --- | --- |
 | **HyperHDR audio** | RTP/L16, port 5004 | the main path; also readable by PulseAudio's `module-rtp-recv` with no custom software |
 | **HyperHDR visualiser** | Flatbuffers images, port 19400 | HyperHDR with nothing installed on the host |
+| **HyperHDR brightness** | JSON-RPC, port 19444 | an existing grabber/ambilight setup — keeps its colour, only pulses brightness |
 | **Raw PCM over UDP** | S16LE datagrams, port 4010 | your own scripts; lowest latency |
 | **Raw PCM over TCP** | S16LE stream, port 4011 | anything that would rather connect than listen |
 | **HTTP WAV** | `http://tv:4012/audio.wav` | opening the TV's audio in VLC |