fix(presence): read the Discord card's play state from playback intent, not the raw render flag

[ajisth69]

What

The Discord card told everyone you were Paused while music was playing.

Root cause

MusicPlayerState.isPlaying is ExoPlayer's raw flag, and ExoPlayer reports it false on every stall — the first buffer of a fresh track, a mid-track network hiccup, the re-prepare behind a quality upgrade — while playWhenReady stays true. The player publishes exactly that as isBuffering (which is playWhenReady-gated). The presence publisher pushed a card on every signature change, and the signature carried the raw flag — so every stall on a streaming connection published "Paused" to Discord, flip-flopping back on resume. On a hi-res stream, stalls are most of the listen.

Fix

The card's play state is now isPlaying || isBuffering — the user's intent at the state boundary (isBuffering only ever rides with playWhenReady, and both flags are cleared together by pause(), a fatal error, and cast teardown):

• buildActivity — timestamps stay up and the asset says play/"Playing" through a stall, so the countdown bar never reads as a frozen paused card mid-buffer.
• signature — a stall shares the playing signature, so a resume repushes nothing (nothing visible changed) and a stall cannot become a frame on its own.
• A genuine pause still reads Paused with no timestamps, and its signature still differs so the pause repushes.

Also logs one line per frame actually handed to Discord, so a card that never appears is diagnosable as no pushes at all, refused pushes (existing WARN), or Discord dropping them (rate limit / app side).

Tests

• bufferingWhileTheUserWantsPlaybackStillReadsAsPlaying — stall card reads Playing, bar up.
• aStallSharesThePlayingSignatureSoItCannotChurnOrBeSwallowed
• aRealPauseStillReadsAsPausedAndRepushes

Run: ./gradlew :app:testDebugUnitTest --tests "com.lastwave.app.presence.DiscordPresenceTest"

Not yet run — if the "sometimes isn't even detecting" symptom persists after this, logcat with -s DiscordPresence DiscordIpcTransport while reproducing will show whether frames stop flowing or Discord refuses them.