Remove the corner helpers akgl_collide_rectangles no longer uses

akgl_rectangle_points, akgl_collide_point_rectangle, akgl_Point and
akgl_RectanglePoints go. They were the intermediate form of an implementation
that changed: akgl_collide_rectangles was eight corner-containment tests built on
them, and it has been four span comparisons since the cross-case fix. Nothing
outside tests/ called either function, and a point-in-rectangle test is four
comparisons a caller can write without a struct conversion in front of them.

akgl_collide_rectangles stays. It has two correct callers in the sidescroller
asking a game-level overlap question -- a coin, a hazard, from an updatefunc --
where a bool is the whole answer and a proxy plus a narrowphase call would be
computing a normal nothing reads. TODO.md records the split rather than leaving
it to be rediscovered.

Public API removal, so 194 exported akgl_ symbols against 196, and the manual's
counts move with them. The perf suite loses its rectangle_points row; the
all-pairs sweep stays as the control it is now labelled, and PERFORMANCE.md says
what 0.8.0 measured against it -- 188.5 us for 32,640 pairs at 256 actors, where
a whole step with collision attached is 54.1 us doing strictly more.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

docs/01-introduction.md

@@ -1,13 +1,15 @@
 # 01. Introduction
 
 libakgl is a C library for building 2D games on SDL3. This is version **0.8.0**. It ships
-195 functions declared across the twenty-three public headers in `include/akgl/`, plus
-`akgl_version()` from the generated `version.h` — 196 exported `akgl_` symbols in
+193 functions declared across the twenty-three public headers in `include/akgl/`, plus
+`akgl_version()` from the generated `version.h` — 194 exported `akgl_` symbols in
 `libakgl.so.0.8`.
 
 **0.8.0 is an ABI break.** The `collide` slot on `akgl_PhysicsBackend` changed signature,
-four public structs grew, and `akgl_physics_arcade_collide` stopped raising `AKERR_API`.
-Collision is the reason; it is [Chapter 15](15-collision.md).
+four public structs grew, `akgl_physics_arcade_collide` stopped raising `AKERR_API`, and
+four symbols were removed — `akgl_rectangle_points`, `akgl_collide_point_rectangle` and the
+two types they used ([Chapter 19](19-utilities.md)). Collision is the reason for all of it;
+it is [Chapter 15](15-collision.md).
 
 It gives you object pools instead of `malloc`, name-based registries instead of pointer
 plumbing, a per-frame tick that updates and draws every live actor, JSON asset formats for
@@ -122,7 +124,7 @@ text you read *is* the header.
 
 ## Reading order
 
-[Chapter 4](04-errors.md) comes before every subsystem chapter, because 183 of those 195
+[Chapter 4](04-errors.md) comes before every subsystem chapter, because 181 of those 193
 functions return `akerr_ErrorContext AKERR_NOIGNORE *` and you cannot read a single example
 until you can read that return value. The dozen that do not are the `void` SDL enumeration
 callbacks and the collision arena's accessors. After that, [Chapter 3](03-getting-started.md) gets a window on