Move BASIC fixtures into the editable language corpus

Move every program and expectation out of tests/reference and register the unified tests/language corpus as local cases. Remove the old immutable-corpus protections from build, maintenance, and documentation paths.

Co-authored-by: andrew <andrew@aklabs.net>

.gitea/workflows/ci.yaml

@@ -17,7 +17,7 @@ jobs:
           # Not recursive, deliberately. The top-level build needs
           # deps/libakerror and deps/libakstdlib, via add_subdirectory, and
           # nothing else: the golden corpus and the Commodore font now live in
-          # this repository (tests/reference/ and assets/fonts/), so
+          # this repository (tests/language/ and assets/fonts/), so
           # deps/basicinterpret is no longer a build dependency at all.
           # It does *not* need deps/libakgl, which is guarded behind
           # AKBASIC_WITH_AKGL and defaults OFF -- and recursing into it would
@@ -62,11 +62,9 @@ jobs:
         run: |
           cmake -S . -B build
           cmake --build build --parallel 2
-      # The suite is 78 cases: 41 golden files byte-compared against the Go
-      # reference's own corpus (checked in at tests/reference/, see its README),
-      # 9 local golden cases for verbs the reference never implemented, 25 unit
-      # tests, 2 embedding examples, and 1 known-failing test that asserts the
-      # *correct* contract for defects carried over from the reference (TODO.md
+      # The suite is 112 cases: 65 language files with sibling expectations,
+      # 43 unit tests, 3 embedding examples, and docs_examples.
+      # Some unit tests assert the *correct* contract for known defects (TODO.md
       # section 6). A green run therefore does not mean defect-free -- see
       # AKBASIC_KNOWN_FAILING_TESTS.
       #