Check the reference corpus in, so the build stops needing the Go submodule

All 41 .bas/.txt pairs copied byte for byte from basicinterpreter@d76162c into
tests/reference/, plus the Commodore font into assets/fonts/ -- the two things
that tied the build to deps/basicinterpret. Both were verified cmp-identical
to the submodule copies.

This reverses a decision that was deliberate and correct at the time: the
corpus was driven in place because copying a submodule's corpus guarantees
drift. Overruled on purpose -- the Go dependency is being deprecated, and a
build that cannot run its acceptance suite without cloning the implementation
it replaced is not finished. tests/reference/README.md records what the drift
now costs and that those expectations are never edited.

Checked rather than assumed: both configurations configure, build and pass
from scratch with deps/basicinterpret moved out of the tree entirely.

The submodule is kept as the behavioural spec, which is a real use. The font
came with an open licence question; assets/fonts/PROVENANCE.md states it.

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

CMakeLists.txt

@@ -195,10 +195,12 @@ if(AKBASIC_WITH_AKGL)
   # The reference's own font, at the reference's own size. Its main.go opens
   # "./fonts/C64_Pro_Mono-STYLE.ttf", which only ever resolved from its own
   # source directory; compiled in, it resolves from anywhere, and AKBASIC_FONT
-  # overrides it at runtime for an installed copy.
+  # overrides it at runtime for an installed copy. Vendored into assets/fonts/
+  # rather than reached for in the submodule -- see assets/fonts/PROVENANCE.md,
+  # which also carries the licence question that came with it.
   target_compile_definitions(basic PRIVATE
     AKBASIC_HAVE_AKGL=1
-    AKBASIC_FONT_PATH="${CMAKE_CURRENT_SOURCE_DIR}/deps/basicinterpret/fonts/C64_Pro_Mono-STYLE.ttf")
+    AKBASIC_FONT_PATH="${CMAKE_CURRENT_SOURCE_DIR}/assets/fonts/C64_Pro_Mono-STYLE.ttf")
 endif()
 akbasic_instrument(basic)
 
@@ -308,7 +310,7 @@ if(AKBASIC_WITH_AKGL)
   target_link_libraries(akbasic_test_akgl_frontend PRIVATE akbasic_frontend)
   target_include_directories(akbasic_test_akgl_frontend PRIVATE "${CMAKE_CURRENT_SOURCE_DIR}/tests")
   target_compile_definitions(akbasic_test_akgl_frontend PRIVATE
-    AKBASIC_TEST_C64_FONT="${CMAKE_CURRENT_SOURCE_DIR}/deps/basicinterpret/fonts/C64_Pro_Mono-STYLE.ttf")
+    AKBASIC_TEST_C64_FONT="${CMAKE_CURRENT_SOURCE_DIR}/assets/fonts/C64_Pro_Mono-STYLE.ttf")
   akbasic_instrument(akbasic_test_akgl_frontend)
   _add_test(NAME akgl_frontend COMMAND akbasic_test_akgl_frontend)
   _set_tests_properties(akgl_frontend PROPERTIES
@@ -331,13 +333,19 @@ if(AKBASIC_WILL_FAIL_TESTS OR AKBASIC_KNOWN_FAILING_TESTS)
   )
 endif()
 
-# The golden-file suite. It drives deps/basicinterpret/tests/**/*.bas in place --
-# the corpus is a submodule and copying it guarantees drift -- and byte-compares
-# stdout against the sibling .txt. One CTest case per .bas so a failure names the
-# file.
+# The reference's own corpus, byte-compared against the sibling .txt. One CTest
+# case per .bas so a failure names the file.
+#
+# It used to be driven in place out of deps/basicinterpret, on the reasoning that
+# copying a submodule's corpus guarantees drift. That reasoning was sound and it
+# has been overruled deliberately: the Go dependency is being deprecated, and a
+# build that cannot run its acceptance suite without cloning the implementation
+# it replaced is not finished. The copy is byte-identical to
+# basicinterpreter@d76162c and tests/reference/README.md records
+# where it came from and what the drift now costs.
 file(GLOB_RECURSE AKBASIC_GOLDEN_CASES
-  RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}/deps/basicinterpret"
-  "${CMAKE_CURRENT_SOURCE_DIR}/deps/basicinterpret/tests/*.bas"
+  RELATIVE "${CMAKE_CURRENT_SOURCE_DIR}/tests/reference"
+  "${CMAKE_CURRENT_SOURCE_DIR}/tests/reference/*.bas"
 )
 
 foreach(_case IN LISTS AKBASIC_GOLDEN_CASES)
@@ -348,7 +356,7 @@ foreach(_case IN LISTS AKBASIC_GOLDEN_CASES)
     NAME golden_${_name}
     COMMAND ${CMAKE_COMMAND}
             -DBASIC=$<TARGET_FILE:basic>
-            -DCASE=${CMAKE_CURRENT_SOURCE_DIR}/deps/basicinterpret/${_case}
+            -DCASE=${CMAKE_CURRENT_SOURCE_DIR}/tests/reference/${_case}
             -P ${CMAKE_CURRENT_SOURCE_DIR}/tests/golden.cmake
   )
 endforeach()
@@ -373,10 +381,12 @@ 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.
+# Still separate now that the reference's corpus lives in this repository too,
+# and the reason changed rather than went away: tests/reference/ is a *record* of
+# what the Go implementation did and nothing in it should ever be edited to suit
+# this one, while tests/language/ is ours to change. Registered under local_ so a
+# failure says at a glance which of the two it came from -- and so a diff that
+# touches tests/reference/ stands out as the thing it is.
 #
 # 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