feat(player): global autoplay & auto-exit option (songs + lessons) (#558)

* fix(v3): pedal click opens the plugin's screen, not its settings

The v3 Pedalboard's settingsTarget() resolved settings-first, so a
plugin that ships both a screen and a settings panel (notably the
bundled Audio Engine) could only ever reach its settings from the
pedalboard — its actual page was unreachable.

Flip to screen-first (stompbox metaphor: step on the pedal, see the
pedal), falling back to settings when there is no screen. Keep a
settings fallback in openPluginSettings() when a declared screen
isn't mounted yet (installing/failed) so settings-bearing plugins are
never stranded on a toast. Drive the pedal aria-label off the same
target so it never promises the wrong surface. Update the unit test
contract to screen > settings > none.

Fixes #555

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* feat(player): global autoplay & auto-exit option (songs + lessons)

Single Settings toggle (autoplayExit, default ON) that auto-starts a song
once it's ready and returns to the launching menu when it ends. Auto-exit
defers while a results/score overlay is on top (heuristic + holdAutoExit()
contract) so a scoring plugin's screen drives the exit. Player origin is now
context-aware (lessons return to the lessons screen via setReturnScreen()),
fixing lesson completion bouncing to the library.

Core-only; songs and lessons share the playSong -> highway path. Adds a
read-only window.slopsmith.autoplayExit getter + holdAutoExit()/setReturnScreen()
for plugins. Unit tests for the pure helpers (_autoplayExitEnabled,
_resolvePlayerOrigin, _resultsOverlayVisible).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/js/speed_reset.test.js

@@ -146,6 +146,9 @@ function loadPlaySong(sandbox) {
         var isPlaying = true;
         var currentFilename = null;
         var _playerOriginScreen = null;
+        var _pendingAutostart = false;
+        function _clearAutoExit() {}
+        function _resolvePlayerOrigin() { return 'home'; }
         function _recordPlaybackBridge() {}
         function _cancelCountIn() {}
         function _resetJuceAudioShimChain() {}