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>

CMakeLists.txt

@@ -107,6 +107,7 @@ set(AKBASIC_SOURCES
   src/runtime.c
   src/runtime_commands.c
   src/runtime_functions.c
+  src/runtime_graphics.c
   src/scanner.c
   src/sink_stdio.c
   src/symtab.c
@@ -191,6 +192,7 @@ set(AKBASIC_TESTS
   environment_scope
   error_codes
   grammar_leaves
+  graphics_verbs
   parser_commands
   parser_expressions
   runtime_evaluate
@@ -269,6 +271,41 @@ if(AKBASIC_GOLDEN_CASES)
   _set_tests_properties(${AKBASIC_GOLDEN_NAMES} PROPERTIES TIMEOUT 30)
 endif()
 
+# The local golden corpus, for verbs the reference never implemented.
+#
+# It has to be separate: the corpus above is a submodule and nothing in this
+# repository 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 says at a glance
+# which corpus it came from.
+#
+# Note what this can and cannot cover. The graphics and sound verbs draw and play
+# rather than print, so what a golden file sees of them is their *refusals* and
+# whatever a program can PRINT about the state they changed. The behaviour that
+# reaches a device is asserted against tests/mockdevice.h instead.
+file(GLOB_RECURSE AKBASIC_LOCAL_CASES
+  RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}/tests"
+  "${CMAKE_CURRENT_SOURCE_DIR}/tests/language/*.bas"
+)
+
+set(AKBASIC_LOCAL_NAMES)
+foreach(_case IN LISTS AKBASIC_LOCAL_CASES)
+  string(REGEX REPLACE "^language/" "" _name "${_case}")
+  string(REGEX REPLACE "\\.bas$" "" _name "${_name}")
+  string(REPLACE "/" "_" _name "${_name}")
+  _add_test(
+    NAME local_${_name}
+    COMMAND ${CMAKE_COMMAND}
+            -DBASIC=$<TARGET_FILE:basic>
+            -DCASE=${CMAKE_CURRENT_SOURCE_DIR}/tests/${_case}
+            -P ${CMAKE_CURRENT_SOURCE_DIR}/tests/golden.cmake
+  )
+  list(APPEND AKBASIC_LOCAL_NAMES local_${_name})
+endforeach()
+
+if(AKBASIC_LOCAL_NAMES)
+  _set_tests_properties(${AKBASIC_LOCAL_NAMES} PROPERTIES TIMEOUT 30)
+endif()
+
 # ---------------------------------------------------------------------------
 # Mutation testing.
 #