refactor(nests): replace hand-built catalog JSON with typed MoqLiteHangCatalog

The two SPEAKER_CATALOG_JSON call sites (MoqLiteNestsSpeaker for the
non-reconnecting path, ReconnectingNestsSpeaker for the JWT-refresh hot-
swap path) both built the manifest as a `\\` + `\"` + `+`-concatenated
string literal. One missed escape and the watcher's parser fails
silently — the broadcast becomes invisible to hang.js with no
indication of why. The two literals had also drifted independently in
review (same content today; nothing prevents drift tomorrow).

Replace both with `MoqLiteHangCatalog.opusMono48k(track).encodeJsonBytes()`,
a Serializable data class that encodes via kotlinx.serialization with
`encodeDefaults = false` + `explicitNulls = false` pinned (so a future
global default-flip can't surface `"bitrate": null` on hang.js's
parser).

The new type lives in :nestsClient because :nestsClient does not depend
on :commons and the parser-side `RoomSpeakerCatalog` (in :commons)
isn't reachable from the publish path. The two classes target the same
wire shape independently; a byte-exact assertion in the new
MoqLiteHangCatalogTest plus the existing RoomSpeakerCatalogTest's
inline-literal round-trip together pin both sides — desync on either
end fails one of the two tests immediately.

https://claude.ai/code/session_014JfZJHSTvyYYWJbC9VbB47

commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/RoomSpeakerCatalogTest.kt

@@ -127,12 +127,13 @@ class RoomSpeakerCatalogTest {
 
     @Test
     fun stripPrefixRoundTripsCanonicalCatalog() {
-        // The catalog payload [com.vitorpamplona.nestsclient.MoqLiteNestsSpeaker]
-        // emits MUST round-trip through the parser — guards against
-        // either side drifting from the kixelated/hang shape.
-        // SPEAKER_CATALOG_JSON lives in nestsClient and isn't
-        // accessible from commons; keep an inline literal that mirrors
-        // it verbatim so a desync triggers this assertion.
+        // The catalog payload `MoqLiteHangCatalog.opusMono48k(...)` emits
+        // (in `:nestsClient`) MUST round-trip through this parser — the
+        // two classes target the same wire shape independently because
+        // `:nestsClient` does not depend on `:commons` and vice versa.
+        // `MoqLiteHangCatalog` isn't reachable from this module; keep an
+        // inline literal that mirrors what the publisher emits verbatim
+        // so a desync on either side trips this assertion.
         val emitted =
             "{\"audio\":{\"renditions\":{\"audio/data\":{" +
                 "\"codec\":\"opus\",\"container\":{\"kind\":\"legacy\"}," +