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/flowcontrol/nestedforloopwaitingforcommand.bas

@@ -0,0 +1,11 @@
+10 REM This shows the waitingForCommand utility in the BasicEnvironment
+11 REM when we have a nested for loop. The inner loop SHOULD execute, but
+12 REM the outer loop should NOT execute. Therefore, neither loop should execute.
+20 FOR I# = 1 TO 0
+25 FOR J# = 2 TO 4
+30 PRINT "waitingForCommand FAILS if this is seen"
+32 QUIT
+35 NEXT J#
+40 NEXT I#
+50 PRINT "SUCCESS"
+80 QUIT