feat: per-voice audio, outlines for what is not an actor, and a coast

Three things a game could not say, each of which had been worked around.

Audio.play_at(id, gain:, pitch:, pan:) fires a one-shot with its own gain,
pitch and stereo position. Audio.volume and Audio.pitch are global - they are
the options screen - so a game placing a sound in the world was fighting them,
and distance attenuation was simply not expressible. The backend already took
volume and rate per call; this adds setPan: alongside them and stops routing
through the master state.

ludic.render3d gains outline_model(model, mat, width, r, g, b): a rim around
something that is not an Actor. The outline pass walked the actor list and
stopped, so instanced scatter - a forest - could not be highlighted at all. It
is a queue flushed by the same pass, which is what gets the depth test right
when the caller does not control pass order.

And terrain_coast(cx, cz, margin, fall), the other way to make an island:
the sea around the survey's own edge rather than cut out of the middle of it.
terrain_island measures a radius from a centre, which drowns two thirds of a
real survey to make an island of the rest; this measures inward from the
boundary, so everything the data covers stays land and the coast is where the
data runs out. Both modes gained a strand - the last few metres of height
either side of the water line compressed, which stretches a cliff plunge out
into beach and shallows.

132 regression tests and the self-host fixpoints pass.

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

examples/library/audio.ludic

@@ -1,7 +1,8 @@
 # audio.ludic — the Audio.* standard library (#22). Sound effects and music over
 # the platform audio backend (AVAudioPlayer on macOS). A sound is loaded once
 # into a handle; sfx fire one-shot, music loops on a single channel; master
-# volume and pitch apply across everything.
+# volume and pitch apply across everything, and Audio.play_at fires one shot with
+# its own gain, pitch and stereo position without touching them.
 #
 # Playback is out-of-band — the audio device is real-time, not part of the
 # deterministic simulation — but every trigger is an ordinary frame-driven call,
@@ -21,6 +22,11 @@ program AudioDemo {
     Audio.volume(0.8)                                # master volume, 0.0..1.0
     Audio.pitch(1.25)                                # playback rate / pitch
     Audio.play(sfx)                                  # one-shot sound effect
+    # the per-voice version: a sound placed in the world rather than the master settings.
+    # This is what distance attenuation is made of - work out how far away and which side
+    # it is, and say so. gain rides on top of the master volume, so the options screen wins.
+    Audio.play_at(id: sfx, gain: 0.35, pitch: 0.9, pan: -0.6)
+    Audio.play_at(sfx, 0.35, 0.9, -0.6)              # or positionally, which a tight loop prefers
     Audio.play_music(mus)                            # looping background music
     print(bi(Audio.is_playing(sfx)))                 # 0 headless