Audio capture and streaming app for webOS 5/6

Captures the TV's audio and sends it out over several transports. The
primary one is HyperHDR: RTP/L16 to a host-side loopback device, since
HyperHDR has no network audio input of its own. A second route renders
the spectrum on the TV and sends FlatBuffers images to port 19400
instead, for setups where touching the host's sound config is not an
option.

  native/       the service: capture backends (PulseAudio, ALSA, exec,
                test tone, all dlopen-based), DSP, and one file per sink
  frontend/     D-pad driven UI at a fixed 1920x1080
  servicefiles/ native service manifest plus the boot script
  host/         RTP receiver and the loopback installer for the HyperHDR
                machine
  tools/        build/package, asset generation, Homebrew Channel
                manifest, on-TV probe
  test/         host-side suites: FlatBuffers and RTP verified against
                real decoders, the engine end to end, the page in jsdom

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

native/src/net/flatbuf.h

@@ -0,0 +1,49 @@
+// A small FlatBuffers builder.
+//
+// HyperHDR's image input speaks the hyperion.ng FlatBuffers schema. The
+// upstream client generates code with flatcc and carries it as a submodule;
+// that is a lot of build machinery for four message shapes, so this
+// implements the builder algorithm directly. It follows the same back-to-front
+// construction as the reference implementations, so buffers it produces are
+// byte-comparable with flatc-generated output (minus vtable deduplication,
+// which is an optimisation, not a format requirement).
+#pragma once
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#define FB_MAX_FIELDS 8
+
+typedef struct {
+    uint8_t* bytes; // full allocation; live data is bytes[head..cap)
+    size_t cap;
+    size_t head;
+    size_t minalign;
+
+    uint16_t vtable[FB_MAX_FIELDS];
+    int vtable_count;
+    size_t object_end; // fb_offset() captured at fb_start_table
+    bool nested;
+    bool failed;
+} fb_t;
+
+bool fb_init(fb_t* b, size_t initial_capacity);
+void fb_free(fb_t* b);
+
+// Offsets are distances from the end of the buffer, matching the reference
+// builders. Zero means "absent".
+uint32_t fb_create_uint8_vector(fb_t* b, const uint8_t* data, size_t count);
+uint32_t fb_create_string(fb_t* b, const char* s);
+
+void fb_start_table(fb_t* b, int num_fields);
+void fb_add_offset(fb_t* b, int slot, uint32_t offset);
+void fb_add_int32(fb_t* b, int slot, int32_t value, int32_t default_value);
+void fb_add_uint8(fb_t* b, int slot, uint8_t value, uint8_t default_value);
+uint32_t fb_end_table(fb_t* b);
+
+void fb_finish(fb_t* b, uint32_t root);
+
+// Valid until the next mutation or fb_free.
+const uint8_t* fb_data(const fb_t* b, size_t* len);
+bool fb_ok(const fb_t* b);