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/README.md

@@ -0,0 +1,49 @@
+# The reference interpreter's own test corpus
+
+These 41 `.bas` files and their 41 `.txt` expectations are **not this project's tests.** They
+are a byte-for-byte copy of the acceptance corpus belonging to the Go implementation this
+interpreter was ported from:
+
+| | |
+|---|---|
+| Source | `https://source.starfort.tech/andrew/basicinterpreter`, `tests/` |
+| Copied at | commit `d76162cb37eeaccddae2cf6cab7af7bc8175d6d2` ("Update README.md") |
+| Copied on | 2026-07-31 |
+| Verified | every file `cmp`-identical to the submodule at the time of the copy |
+
+They are registered as the `golden_*` CTest cases and run against `basic` on every build, in
+both the default and the `-DAKBASIC_WITH_AKGL=ON` configurations.
+
+## Why they are a copy now, when they deliberately were not
+
+Until 2026-07-31 these were driven *in place* out of `deps/basicinterpret`, and the reasoning
+was written down in `CMakeLists.txt`: the corpus is a submodule, and copying a submodule's
+corpus guarantees drift.
+
+That reasoning was sound. It was overruled on purpose, because the Go dependency is being
+deprecated and a build that cannot run its own acceptance suite without cloning the
+implementation it replaced is not finished.
+
+**What the change costs, stated plainly so nobody has to rediscover it:** if the Go
+implementation ever gains a test, this directory will not. There is no longer anything
+watching for that. The trade is deliberate — the Go project is the thing being retired, so a
+corpus that stops tracking it is a corpus that stopped tracking something that stopped moving.
+
+## The rule for editing these
+
+**Don't.**
+
+This directory is a *record of what the Go implementation did*, and it is what makes the claim
+"reproduces the reference byte for byte" checkable at all. A failure here means this
+interpreter's behaviour changed, and the answer is to change this interpreter or to record a
+deliberate deviation in `TODO.md` §5 — never to edit the expectation to match.
+
+New cases for this project's own behaviour go in `tests/language/`, which is registered under
+`local_*` for exactly this reason: the prefix says which corpus a failure came from, and a diff
+that touches `tests/reference/` stands out as the thing it is.
+
+There is one case worth knowing about before anybody "fixes" it.
+`examples/strreverse.bas` names a variable `INPUT$`, which a real Commodore 128 would refuse
+and which this interpreter accepts only because the reference's reserved-word check never
+fires. That is `TODO.md` §6 item 16, and it is the reason that item is filed as a decision
+rather than a defect: making the check work costs this case.