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>

docs/language/audio/audio-play_at.md

@@ -0,0 +1,42 @@
+---
+id: audio-play_at
+name: Audio.play_at
+category: audio
+kind: namespace-method
+tokens: Audio.play_at
+sig: Audio.play_at(id, gain, pitch, pan) -> void
+tip: Fire a one-shot with its own gain, pitch and stereo position.
+order: 2
+ns: Audio
+member: play_at
+---
+
+Plays the sound <code>id</code> from the start as a one-shot, with its **own** gain, pitch
+and stereo position, leaving the master settings alone.
+
+<code>Audio.volume</code> and <code>Audio.pitch</code> are global: they exist so a player
+can turn the game down, and a game that used them to place a sound in the world would be
+fighting its own options screen. This is the per-voice version, and it is what distance
+attenuation is made of — work out how far away a sound is and which side it is on, and say
+so here.
+
+- <code>gain</code> — 0.0 to 1.0, multiplied by the master volume, so the options screen
+  still wins.
+- <code>pitch</code> — 1.0 is as recorded; 0.5 an octave down, 2.0 an octave up. Clamped
+  to the backend's 0.25 .. 4.0.
+- <code>pan</code> — -1.0 hard left, 0.0 centred, 1.0 hard right.
+
+```ludic
+program Demo {
+  entry {
+    let call = Audio.load("elk.wav")
+    # an elk eighty metres off, over your left shoulder
+    Audio.play_at(id: call, gain: 0.3, pitch: 0.96, pan: -0.55)
+  }
+}
+```
+
+One honest limitation, and it is the backend's: a loaded sound is one player, so firing
+the same handle again restarts it rather than layering a second copy over the first. Load
+a handle per variant when several need to overlap — which is what a game does anyway, to
+stop a repeated sound machine-gunning.