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>

tests/reference/language/label.bas

@@ -0,0 +1,5 @@
+10 I# = 0
+20 LABEL DOITAGAIN
+30 I# = I# + 1
+40 IF I# <> 3 THEN GOTO DOITAGAIN
+50 PRINT "DONE"