Say what was hit, which way is out, and how far

The narrowphase has been producing a contact since it went in, and the interpreter
was throwing it away. `RCOLLISION(n, f)` reports it: what was hit (a sprite or a
`SOLID` rectangle), which one, the contact normal, the penetration depth, the
contact point, and which axis to reverse.

**The normal points out of the other thing and toward this one**, so a program
moves along it by the depth and is exactly clear. That sign is the one assertion
in the new test that could not be caught any other way -- both parties of a
sprite-against-sprite hit get their own record, each pointing the way *that*
sprite has to move, and sharing one would tell both to go the same direction,
which is how two things end up stuck inside each other.

**Field 7 is the one that deletes the most BASIC.** It is the minimum translation
axis, computed from the normal in C, and it is there because doing it in BASIC
means comparing two floats -- which is exactly where this dialect's left-operand
rule catches people. `BALLBRICKS`/`TESTCELL` in the artwork breakout spend six
lines computing an overlap rectangle and comparing its width to its height to get
this number.

The record is **sticky and deepest-wins**: replaced whenever that sprite is in a
contact and otherwise left alone, so `BUMP` stays the event and this stays the
detail of it. Making it clear itself when nothing touches would break the pairing,
because `BUMP` accumulates across steps and a once-a-frame poll would find the
detail already gone. Reading `BUMP` clears both, so they cannot disagree.

Deliberately narrower than `akgl_Contact`: no actor pointers, because BASIC has no
actor; no tile fields, because there is no tilemap; no z, because every test is
planar; and **no `dt` and no `sensor`**, which libakgl documents as filled in by
the resolver. This interpreter never resolves anything, so those two come back
zero and mean nothing, and an always-zero field in a reference table is a lie.

Documented with the two caveats that matter: fields 2, 3 and 4 are floats and want
a `%` variable, and the contact *point* is exact only for two boxes -- libakgl's
solver returns a point on the portal it converged to, while the normal and depth
are exact for every pair.

Chapter 8's collision section stops claiming only type 1 exists, which has been
false since the previous commit.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwxGB6TdoVvZ11KQQME9cL

docs/12-function-reference.md

@@ -25,6 +25,7 @@ so a call with the wrong number is a syntax error rather than a surprise.
 | `POINTER` | 1 | `POINTER(V)` | The address of a variable's value. |
 | `POINTERVAR` | 1 | `POINTERVAR(V)` | The address of the variable structure itself, metadata included. |
 | `RAD` | 1 | `RAD(n)` | Degrees converted to radians. |
+| `RCOLLISION` | 2 | `RCOLLISION(n, f)` | What sprite `n` last collided with and how: what, which, the normal, the depth, the point, and which axis to reverse. See Chapter 8. |
 | `RGR` | 1 | `RGR(f)` | The `GRAPHIC` mode (0), the drawing surface's width (1) or height (2) in pixels, or a character cell's width (3) or height (4). |
 | `RIGHT` | 2 | `RIGHT(A$, n)` | The rightmost `n` characters. Clamped. |
 | `RWINDOW` | 1 | `RWINDOW(f)` | The current text window's rows (0) or columns (1). Field 2 is a C128 screen mode and is refused. |