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>

assets/fonts/PROVENANCE.md

@@ -0,0 +1,41 @@
+# C64_Pro_Mono-STYLE.ttf
+
+| | |
+|---|---|
+| Font | C64 Pro Mono, by STYLE — <https://style64.org/c64-truetype> |
+| Copied from | `basicinterpreter@d76162c`, `fonts/C64_Pro_Mono-STYLE.ttf` |
+| Copied on | 2026-07-31 |
+| Verified | `cmp`-identical to the submodule copy |
+
+## Why it is here
+
+It is the font `deps/basicinterpret/main.go` opens, at the size it opens it, and reproducing
+that is part of what the SDL frontend is for. It was reached for inside the submodule until the
+Go dependency started being deprecated; the build now has no reason to clone that repository,
+and this was the last thing tying it to one.
+
+Two things use it, both through compile definitions in `CMakeLists.txt`:
+
+- `AKBASIC_FONT_PATH` — what an `-DAKBASIC_WITH_AKGL=ON` build of `basic` opens. Overridable at
+  runtime with the `AKBASIC_FONT` environment variable, which is what an installed copy or a
+  different font wants.
+- `AKBASIC_TEST_C64_FONT` — what `tests/akgl_frontend.c` opens, because `TODO.md` §3's
+  acceptance asks the drawn text to be proved in *this* font rather than any monospaced one.
+
+## The licence question, which is open
+
+**STYLE's C64 TrueType fonts are not public domain and this repository does not carry their
+licence.** Neither did `basicinterpreter`, which is where the file came from. The terms at
+<https://style64.org/c64-truetype/license> permit free personal and non-commercial use and
+require the licence to accompany redistribution; commercial redistribution needs their
+permission.
+
+Copying it here changes nothing legally — the same file was already being redistributed from
+the same owner's other public repository — but it does move the question into a project that
+is meant to be embeddable in other people's games, which is where it starts to matter.
+
+**Before this is shipped in anything commercial, or vendored into a game that is:** either
+obtain STYLE's permission and include their licence file beside this one, or replace the font.
+Replacing it is cheap — the frontend measures whatever font it is handed
+(`akbasic_sink_init_akgl` derives the character grid from `akgl_text_measure`), so any
+monospaced TTF works and only the two compile definitions above would change.