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>

README.md

@@ -1,4 +1,4 @@
-This BASIC is styled after [Commodore BASIC 7.0](http://www.jbrain.com/pub/cbm/manuals/128/C128PRG.pdf) and the [Dartmouth BASIC from 1964](https://www.dartmouth.edu/basicfifty/basic.html). It is a C rewrite of [basicinterpreter](https://source.starfort.tech/andrew/basicinterpreter), which was itself built from the instructions for the Java implementation of Lox in [craftinginterpreters.com](https://craftinginterpreters.com) before striking off on its own. The Go version is vendored at `deps/basicinterpret` and is the behavioural specification: when a question about semantics comes up, the answer lives in that code and in its `tests/`.
+This BASIC is styled after [Commodore BASIC 7.0](http://www.jbrain.com/pub/cbm/manuals/128/C128PRG.pdf) and the [Dartmouth BASIC from 1964](https://www.dartmouth.edu/basicfifty/basic.html). It is a C rewrite of [basicinterpreter](https://source.starfort.tech/andrew/basicinterpreter), which was itself built from the instructions for the Java implementation of Lox in [craftinginterpreters.com](https://craftinginterpreters.com) before striking off on its own. The Go version is vendored at `deps/basicinterpret` and is the behavioural specification: when a question about semantics comes up, the answer lives in that code. Its acceptance corpus is **checked in here** at [`tests/reference/`](tests/reference/README.md) and runs on every build, so nothing about building or testing this project needs that submodule any more.
 
 ```sh
 git submodule update --init --recursive
@@ -9,7 +9,7 @@ cmake --build build --parallel
 ./build/basic
 
 # To run a basic file from the command line
-./build/basic deps/basicinterpret/tests/language/functions.bas
+./build/basic tests/reference/language/functions.bas
 
 # The test suite: unit tests plus the Go version's own corpus, byte-compared
 ctest --test-dir build --output-on-failure
@@ -25,7 +25,7 @@ ctest --test-dir build-akgl --output-on-failure
 
 # That build of `basic` is a different program: it opens an 800x600 window, draws
 # BASIC output into it in the Commodore font, and still puts every byte on stdout.
-./build-akgl/basic deps/basicinterpret/tests/language/functions.bas
+./build-akgl/basic tests/reference/language/functions.bas
 ```
 
 There are two workflows. **`.gitea/workflows/ci.yaml`** runs on every push: the suite,
@@ -72,7 +72,9 @@ And the port is a good excuse to find out what the original actually does, as op
 
 # What Works?
 
-Everything the Go version does. All 41 `.bas` files in `deps/basicinterpret/tests/` produce **byte-identical** output from both implementations, including error messages and the trailing blank line after one. Those files are driven in place as individual CTest cases rather than copied, so the corpus cannot drift from upstream.
+Everything the Go version does. All 41 `.bas` files of the reference's own corpus produce **byte-identical** output from both implementations, including error messages and the trailing blank line after one. Each is a separate CTest case, so a failure names the file.
+
+That corpus lives at [`tests/reference/`](tests/reference/README.md), copied byte for byte out of the Go project. It was driven in place out of the submodule until the Go dependency started being deprecated; its README records where it came from, and that nobody is watching for drift any more because the thing it tracked has stopped moving. **Do not edit those expectations** — a failure there means this interpreter's behaviour changed.
 
 ## Case Sensitivity
 
@@ -461,7 +463,7 @@ The exception is `FILTER`, which sets the SID's filter cutoff, band switches and
 * [libakerror](https://source.starfort.tech/andrew/libakerror) 1.0.0 — TRY/CATCH-style error contexts. Every function that can fail returns one.
 * [libakstdlib](https://source.starfort.tech/andrew/libakstdlib) 0.2.0 — libc wrappers that report through `libakerror`.
 * [libakgl](https://source.starfort.tech/andrew/libakgl) 0.2.0 — **optional**, only for `-DAKBASIC_WITH_AKGL=ON`. Pulls in SDL3, and requires `libakstdlib` 0.2, which is why the two move together.
-* [basicinterpret](https://source.starfort.tech/andrew/basicinterpret) — the Go original, vendored as the behavioural spec and the acceptance corpus. Not linked, not built.
+* [basicinterpret](https://source.starfort.tech/andrew/basicinterpret) — the Go original. Vendored as the behavioural spec for questions about semantics, and for nothing else: its acceptance corpus and its Commodore font are checked in here now, so **the build and the test suite do not need it**. Not linked, not built, and safe to omit.
 
 Everything is a submodule; `git submodule update --init --recursive` gets all of it. There is nothing to install first.