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>

.gitea/workflows/release.yaml

@@ -66,9 +66,9 @@ jobs:
         uses: actions/checkout@v4
         with:
           # The harness copies the repo and configures a build inside the copy,
-          # so it needs the same submodules the main build does -- including
-          # deps/basicinterpret, because the golden corpus is part of what kills
-          # mutants.
+          # so it needs the same submodules the main build does. The golden
+          # corpus is part of what kills mutants and it is checked in now, so
+          # that is libakerror and libakstdlib and nothing else.
           submodules: true
       - name: dependencies
         run: |