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>

scripts/mutation_test.py

@@ -282,9 +282,10 @@ def copy_tree(src, dst):
     # and a coverage tree drags along every .gcno/.gcda as well.
     #
     # deps/ is copied wholesale and has to be: the build pulls libakerror and
-    # libakstdlib in with add_subdirectory, and the golden suite drives the .bas
-    # corpus in deps/basicinterpret in place. Only the reference's vendored
-    # Windows DLLs are dead weight, hence "*.dll".
+    # libakstdlib in with add_subdirectory. The golden corpus used to be the
+    # other reason -- it was driven in place out of deps/basicinterpret -- and
+    # now lives in tests/reference/, which comes along with the rest of the tree.
+    # Only vendored Windows DLLs are dead weight, hence "*.dll".
     ignore = shutil.ignore_patterns("build*", ".git", "*.o", "*.so", "*~",
                                     "#*#", "*.iso", "*.png", "*.gcno", "*.gcda",
                                     "*.dll")