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>

selfhost/backend/emit_head.ludic

@@ -115,6 +115,7 @@ function emit_header() -> void {
   emith("declare i32 @snd_playing(ptr)\n")
   emith("declare void @snd_set_volume(ptr, i32)\n")
   emith("declare void @snd_set_rate(ptr, i32)\n")
+  emith("declare void @snd_set_pan(ptr, i32)\n")
   emith("declare ptr @hs_req_new(ptr, ptr)\n")          # #6 HTTP transport (http.ll)
   emith("declare void @hs_req_header(ptr, ptr, ptr)\n")
   emith("declare void @hs_req_body(ptr, ptr, i32)\n")