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/04-errors.md

@@ -14,7 +14,7 @@ This manual does not restate them, because a copy here would be wrong the day li
 changes and nothing in this repository would notice.
 
 What is genuinely libakgl's, and is written down nowhere else, is **which statuses these
-195 functions raise and what each one means here**. That is the three tables below.
+193 functions raise and what each one means here**. That is the three tables below.
 
 The house rules for *writing* code against the protocol — never a `*_RETURN` inside an
 `ATTEMPT`, never a bare `return` out of a `HANDLE`, `CLEANUP` before `PROCESS`, an