Pool collision proxies, and tie one to the life of its actor

A proxy is what the broad phase will index: an actor's shape, where it is, and
the bounds that follow from those. It is a sixth heap layer, two per actor --
one for the actor itself and headroom for static geometry a game registers that
is not tile-aligned. Solid *tiles* deliberately get none: the broad phase will
read those out of the tilemap's own array, because one proxy per tile is tens of
megabytes to index data that is already a grid.

The pool follows the existing convention rather than improving on it.
akgl_heap_next_collision_proxy finds a free slot and does not claim it;
akgl_collision_proxy_initialize claims it. That is TODO.md item 8's asymmetry,
and it is kept on purpose: an acquire abandoned before initialization leaks
nothing, because the slot still reads as free, and a sixth pool with its own
rule would be worse than one defect with five instances. Fixing item 8 means
fixing all five together with every call site audited, and that is its own
change.

The proxy holds a *copy* of the shape rather than a pointer. An actor's own
logic may rewrite its shape mid-step -- a character crouching, a projectile
arming -- and a broad phase whose bounds came from a shape that has since
changed is a bug that only appears when two things happen in the same frame.

akgl_heap_release_actor now releases the actor's proxy. Without it the proxy
holds a borrowed `owner` into a slot that has just been zeroed, so the broad
phase keeps a registration whose owner reads as a free actor, and the next
contact against it is a collision with nothing. Verified by removing the release
and watching test_proxy_dies_with_its_actor go red.

The tests pin the convention as well as the behaviour: that two acquires with no
initialize between them return the same slot is asserted, not merely tolerated,
so that changing the convention has to change a test that says why it exists.

Co-Authored-By: Claude Code <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

include/akgl/actor.h

@@ -124,6 +124,7 @@ typedef struct akgl_Actor {
     SDL_Time movetimer;                     /**< Unused. Nothing in the library reads or writes it. */
     akgl_CollisionShape shape;              /**< The collision volume this actor occupies. Copied from the character by akgl_actor_set_character unless #shape_override is set. Zeroed by akgl_actor_initialize, so an actor nobody configured does not collide. */
     bool shape_override;                    /**< The game set #shape itself and akgl_actor_set_character must leave it alone. Distinct from "the shape is empty": an actor deliberately given no collider and an actor not yet given one are different states, and only this flag tells them apart. */
+    struct akgl_CollisionProxy *proxy;      /**< This actor's registration in the broad phase, or `NULL` when it has none. Owned by the library and released with the actor; a game neither sets nor frees it. */
     // Velocity. Combined effect of all forces acting on the actor resulting
     // in energy along an axis.
     float32_t vx;   /**< Velocity along x, world units per second. Recomputed each step as `ex + tx`; writing it directly is overwritten. */