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>

changes/audio-play-at.md

@@ -0,0 +1,14 @@
+bump: minor
+type: feat
+**`Audio.play_at(id, gain:, pitch:, pan:)`** — fire a one-shot with its own gain, pitch
+and stereo position, leaving the master settings alone.
+
+`Audio.volume` and `Audio.pitch` are global: they are there 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 — a 3D game works out how far away a sound is and which side it is on, and says so.
+`gain` rides on top of the master volume, so the options screen still wins; `pan` runs
+-1 (hard left) to 1 (hard right).
+
+A loaded sound is still one player, so firing the same handle again restarts it rather
+than layering a second copy. Load a handle per variant when several need to overlap.

changes/outline-model.md

@@ -0,0 +1,16 @@
+bump: minor
+type: feat
+**`ludic.render3d`: `outline_model(model, mat, width, r, g, b)`** — a rim around something
+that is not an Actor.
+
+An Actor has had an `outline` since the outline pass landed, and everything else in a
+scene had nothing: the pass walked the actor list and stopped. Instanced scatter was the
+gap that mattered, because a game that highlights whatever the crosshair is on could
+highlight every object in the world *except* the twenty thousand most common ones — the
+trees.
+
+`outline_model` queues a model at a transform from anywhere in the frame and the outline
+pass flushes it alongside the actors', which is what gets the depth test right: the rim has
+to be drawn after the scene it is tested against, and a caller does not control pass order.
+A layer whose vertex shader moves its instances — wind, or standing them on the drawn
+terrain — should be handed the transform that shader arrives at.

changes/terrain-coast.md

@@ -0,0 +1,17 @@
+bump: minor
+type: feat
+**`ludic.render3d`: `terrain_coast(cx, cz, margin, fall)`** — the other way to make an
+island, and the one a real survey wants: the sea goes around the survey's **own edge**
+rather than being cut out of the middle of it.
+
+`terrain_island` measures a radius out from a centre, which suits a made-up map and
+drowns most of a real one — an 8 km mountain survey loses two thirds of itself to make an
+island of the rest. `terrain_coast` measures inward from the boundary instead: everything
+the data covers stays land, the outer `margin` metres go under water, and the `fall` metres
+inside that are scaled down into it. The band is wobbled by low-frequency noise, so what
+comes out is headlands and bays rather than the square the data arrived in.
+
+Both modes also gained a **strand**. Scaling alone hands a 500 m mountainside a 40-degree
+plunge into the sea, which is a cliff coast and nothing else; the last few metres of height
+either side of the water line are now compressed, which stretches them out horizontally
+into beach and shallows. Inland lakes are untouched — they have their own bed.