Compiler: $EXTERN pragma for calling C functions from BASIC (Level 2 FFI) (#1)

Add a '$EXTERN NAME(ARGTYPES) AS RET pragma so compiled BASIC can call C
functions directly, the natural follow-up to Level 1 (--emit-obj /
--main-name). The pragma is an apostrophe comment, so the interpreter
ignores it while the compiler registers it.

Map INTEGER/SINGLE/DOUBLE/STRING to int16_t/float/double/const char* at the
boundary: a string argument crosses as a temporary C copy that is freed
after the call, and a string return is copied into the pool. The call name
is matched case-insensitively but emitted as the C symbol with the case
written in the pragma. Names are recognized before parse_var() truncates
identifiers to two significant characters, so multi-character C function
names work.

A string return that aliases a char* argument is copied before the argument
temporaries are freed, which avoids a use-after-free. Over-supplied
arguments are consumed without desyncing the token stream and warn on arity
mismatch.

Docs: getting-started.md "Foreign Functions from BASIC". Test:
tests/run_ffi_test.sh, wired into CI. 63/63 compiler, 72/72 interpreter,
68/68 compat still pass.

Also refile the roadmap "Next Up" backlog as git-bug issues and prune
docs/roadmap.md to point at git-bug as the source of truth for planned work.

Co-authored-by: Eremey Valetov <evvaletov@users.noreply.github.com>

.github/workflows/ci.yml

@@ -30,6 +30,9 @@ jobs:
       - name: Compiler tests
         run: bash tests/run_compiler_tests.sh
 
+      - name: Cross-language (FFI) test
+        run: bash tests/run_ffi_test.sh
+
   dos-cross-compile:
     runs-on: ubuntu-latest
     steps: