Let a program say what part of a sprite collides

`SPRHIT n, kind [,x1, y1, x2, y2]` gives a sprite a collision shape: a box, a
circle inscribed in it, or a capsule. `RSPHIT(n, f)` reads it back in SPRHIT's
own argument order, the way RSPRITE and RSPPOS already do, and needs no device
because it answers from interpreter state.

The rectangle is two corners measured from the sprite's top-left, in device
pixels -- the same `x1, y1, x2, y2` that `BOX` and `SSHAPE` take. A dialect with
two spellings for a rectangle is one nobody can write from memory. Omit it and
the shape fits whatever the picture turned out to be, which is what a sprite
loaded from a file needs: `SPRSAV "ship.png", 1` takes the image's own size and
the program never learns what that was.

**A sprite nobody has shaped collides with its whole frame, expansion bits
included, exactly as before.** That is a promise rather than a convenience, and
it has its own test: the same two sprites in the same two places, once with no
SPRHIT and once with a four-pixel box, reporting a collision and then not.

Named SPRHIT rather than SPRSHAPE because "shape" already means "a region SSHAPE
saved" in this dialect, in this very chapter -- `SPRSAV A$, 1` takes one -- and a
reader who typed `SPRSHAPE A$, 1` would have had every reason to. Both names, and
RSPHIT, were grepped against every label in docs/, examples/ and both corpora
first: a bare word is a label here, so a verb and a label share one namespace and
taking a name a checked-in listing already uses would break it silently.

`SPRHIT n, 0` takes a sprite out of collision while leaving it on the screen --
the ghost, the flashing invulnerable player, the pickup already taken. Hiding it
with `SPRITE n, 0` stops it colliding too, and is what you want when it should
not be seen either.

The circle answers the complaint chapter 8 already ships a figure of. That figure
shows two discs whose *boxes* touch at a corner while the artwork is nowhere
near, and `BUMP(1)` reporting a collision; two `SPRHIT n, 2` and it stops. The
test asserts both halves so the figure's caption stays true.

`tests/verbs_table.c` caught RSPHIT filed after RSPPOS rather than before it,
which is the sorted-table test doing exactly the job it exists for.

Docs: a new section in chapter 8, rows in the verb and function references in
alphabetical order, and chapter 13's "collision is by bounding box" becomes
"collision is by shape" with the addition named. 111 with akgl, 110 without, and
the artwork breakout still runs clean.

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

include/akbasic/akgl.h

@@ -272,6 +272,22 @@ typedef struct
      * this is compared rather than flagged.
      */
     SDL_FRect syncedbox[AKBASIC_MAX_SPRITES];
+
+    /**
+     * SPRHIT's shape per slot, as the device was told it.
+     *
+     * A second copy of what akbasic_Sprite already carries, and deliberately: a
+     * device is told about changes and never asked what they were
+     * (include/akbasic/sprite.h), so the backend has to be able to answer from
+     * its own state rather than reaching back into the runtime it does not have
+     * a pointer to.
+     */
+    int shapekind[AKBASIC_MAX_SPRITES];
+    float32_t shapex1[AKBASIC_MAX_SPRITES];
+    float32_t shapey1[AKBASIC_MAX_SPRITES];
+    float32_t shapex2[AKBASIC_MAX_SPRITES];
+    float32_t shapey2[AKBASIC_MAX_SPRITES];
+    bool shapeexplicit[AKBASIC_MAX_SPRITES];
 } akbasic_AkglSprites;
 
 /**

include/akbasic/sprite.h

@@ -37,6 +37,21 @@
 /** @brief Sprites a program has. Eight, as on a C128; the verbs number them 1-8. */
 #define AKBASIC_MAX_SPRITES		8
 
+/**
+ * @brief The collision shapes SPRHIT understands.
+ *
+ * **Deliberately libakgl's own numbers**, so the two documents agree and nobody
+ * has to maintain a mapping between them. A kind this interpreter has no use for
+ * is still refused by name rather than silently accepted.
+ */
+#define AKBASIC_SHAPE_NONE		0
+#define AKBASIC_SHAPE_BOX		1
+#define AKBASIC_SHAPE_CIRCLE		2
+#define AKBASIC_SHAPE_CAPSULE_X		3
+#define AKBASIC_SHAPE_CAPSULE_Y		4
+/** @brief One past the last kind, for the range check. */
+#define AKBASIC_SHAPE_LAST		5
+
 /** @brief Width of a Commodore sprite in pixels. */
 #define AKBASIC_SPRITE_WIDTH		24
 /** @brief Height of a Commodore sprite in pixels. */
@@ -87,6 +102,23 @@ typedef struct
     double y;
     double angle;		/* MOVSPR's continuous motion: degrees		*/
     int speed;			/*   clockwise from vertical, and 0-15		*/
+
+    /**
+     * SPRHIT's collision shape: one of the #AKBASIC_SHAPE_* kinds, and a
+     * rectangle measured from the sprite's top-left corner.
+     *
+     * `shapeexplicit` is what separates "the program asked for the whole frame"
+     * from "the program has not said". They are the same shape today and will
+     * not stay that way: a frame-fitting shape has to be rebuilt when the
+     * picture or an expansion bit changes, and one the program gave in pixels
+     * must not be.
+     */
+    int shapekind;
+    double shapex1;
+    double shapey1;
+    double shapex2;
+    double shapey2;
+    bool shapeexplicit;
 } akbasic_Sprite;
 
 /**
@@ -181,6 +213,19 @@ typedef struct akbasic_SpriteBackend
      * remember anything.
      */
     akerr_ErrorContext AKERR_NOIGNORE *(*collisions)(struct akbasic_SpriteBackend *self, uint16_t *mask);
+    /**
+     * Give sprite @p n a collision shape.
+     *
+     * @p kind is an #AKBASIC_SHAPE_NONE .. #AKBASIC_SHAPE_CAPSULE_Y, and the
+     * rectangle is two corners measured from the sprite's top-left, in device
+     * pixels -- the same form `BOX` and `SSHAPE` take, because a dialect with two
+     * spellings for a rectangle is a dialect nobody can write from memory.
+     *
+     * Optional. A backend that withholds it makes `SPRHIT` refuse by name, which
+     * is what a device that cannot do what was asked is supposed to do.
+     */
+    akerr_ErrorContext AKERR_NOIGNORE *(*shape)(struct akbasic_SpriteBackend *self, int n, int kind,
+						double x1, double y1, double x2, double y2);
 } akbasic_SpriteBackend;
 
 /**