Draw: implement the BASIC 7.0 graphics verbs

GRAPHIC, COLOR, DRAW, BOX, CIRCLE, PAINT, SCALE, SSHAPE, GSHAPE and LOCATE, all
against the akbasic_GraphicsBackend record rather than akgl_draw_* directly, so
src/runtime_graphics.c includes no SDL and the whole group is testable in a build
with no SDL on the machine.

The reference lists every one of these as unimplemented, so the semantics come
from Commodore BASIC 7.0 rather than from a port, and four places where a modern
renderer cannot do what a C128 did are recorded in TODO.md section 5 rather than
silently substituted:

- CIRCLE is drawn as a polygon of inc-degree segments and akgl_draw_circle is
  deliberately unused. 7.0's CIRCLE takes two radii, an arc range and a rotation,
  so the primitive could serve only the fully-defaulted call, and a shape that
  changed character depending on whether the radii happened to be equal would be
  worse than one uniformly a polygon.
- SSHAPE puts a SHAPE:<n> handle in the string variable rather than the pixels,
  because a value's string is a fixed 256 bytes and a region is a device surface.
  GSHAPE refuses a string without that prefix instead of parsing whatever digits
  it finds and pasting an unrelated slot.
- BOX fills on a negative angle; 7.0 puts the fill flag after the rotation, which
  would make a filled box a seventh argument.
- GRAPHIC stores its mode and honours only the one consequence that means
  anything here -- mode 0 is text -- while still refusing an out-of-range mode,
  since that is a typo worth catching.

PAINT surfaces the flood fill's AKERR_OUTOFBOUNDS as an error rather than
success. The device gives up when its span stack runs out having filled *part* of
the region, and a program that cannot tell that happened cannot recover from it.
Note the shape of that handler: HANDLE sets handled = true on the context, so a
FAIL_RETURN from inside the HANDLE block hands the caller something already
marked handled, whose FINISH_LOGIC then declines to pass it up and releases it --
the error disappears and PAINT reports success. Flag inside the block, raise
after FINISH.

COLOR, LOCATE and SCALE need no device on purpose, so a program can set itself up
before a host has lent it a renderer.

Adds a second golden corpus under tests/language/. The corpus in
deps/basicinterpret is a submodule and nothing here may add files to it, but
goal 2's new verbs still need the .bas/.txt half of their coverage. Registered
under local_ so a failure names which corpus it came from. What it can cover is
limited -- these verbs draw rather than print -- so the behaviour that reaches a
device is asserted against tests/mockdevice.h instead.

65/65 ctest, clean under -Wall -Wextra, doxygen clean.

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

tests/language/graphics/no_device.txt

@@ -0,0 +1,3 @@
+STATE VERBS OK
+? 80 : RUNTIME ERROR DRAW needs a graphics device and this runtime has none
+