fix(playback): restore audible volume when M unmutes at zero volume

Addresses the Codex review finding on #1398: after arrowing the volume
down to zero (which mutes), M flipped muted off while leaving the volume
at 0, so the player looked unmuted but stayed silent — in all three
legacy engine adapters.

Mirror the shared controls' ControlsVolume semantics with a per-adapter
LegacyMuteMemory: muting remembers the audible volume, and unmuting while
the volume sits at zero restores it, with the same 0.5 fallback when
nothing was remembered.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014h2cZi5DcFSbcmV7WgB6qB

docs/architecture/player-controls-contract.md

@@ -362,7 +362,10 @@ Shared legacy rules: seek is gated on authoritative `isLive` plus a finite,
 positive duration (ArtPlayer gates on `art.duration`, the same value its seek
 setter clamps against, so an unknown duration never jumps to zero); volume
 steps by ±5% and syncs muted state the way `applyVideoVolume` does (raising
-out of mute unmutes, reaching zero mutes); `isAvailable` is the host's
+out of mute unmutes, reaching zero mutes); M mirrors `ControlsVolume`'s mute
+memory through `LegacyMuteMemory` — muting remembers the audible volume, and
+unmuting while the volume sits at zero restores it (same 0.5 fallback), so M
+can never leave the player silently "unmuted"; `isAvailable` is the host's
 `interactionEnabled`, so a visible playback diagnostic disables the keys; and
 Escape defaults to a no-op without consuming the key, because the vendor
 chrome owns its own overlays. Instances attach in the component's legacy