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/examples/bubblesort.bas

@@ -0,0 +1,29 @@
+10 DIM A#(5)
+15 ITERATIONS# = 0 
+20 A#(0) = 5
+21 A#(1) = 2
+22 A#(2) = 4
+23 A#(3) = 1
+24 A#(4) = 3
+25 PRINT "BEFORE SORTING: "
+26 FOR I# = 0 TO 4
+27     PRINT A#(I#)
+28 NEXT I#
+30 CHANGED# = 0
+32 ITERATIONS# = ITERATIONS# + 1
+35 FOR I# = 0 TO 3
+45     J# = I#+1
+46     REM PRINT "COMPARING INDEXES " + I# + " (" + A#(I#) + ") AND " + J# + "
+50     IF A#(I#) <= A#(J#) THEN GOTO 100
+55     REM PRINT "SWAPPING INDEXES " + I# + " AND " + J#
+60     TMP# = A#(I#)
+70     A#(I#) = A#(J#)
+80     A#(J#) = TMP#
+85     CHANGED# = CHANGED# + 1
+100 NEXT I#
+110 IF CHANGED# <> 0 THEN GOTO 30
+115 PRINT "AFTER SORTING:"
+120 FOR I# = 0 TO 4
+130   PRINT A#(I#)
+140 NEXT I#
+145 PRINT "SORTED IN " + ITERATIONS# + " ITERATIONS"