Add DOS build script, compiler/DOS test harnesses, FreeDOS package, CI

- build_dos.sh: Linux-friendly cross-compile to DOS via OpenWatcom V2.
  OpenWatcom's wmake on Linux can't apply the .c.obj implicit rule for
  subdirectory paths, and Makefile.dos / Makefile.dos16 rely on DOS-
  only commands like 'del'.  Script invokes wcc / wcc386 directly,
  tracks 16-bit vs 32-bit mode via a stamp file (auto-cleans on
  switch), generates a wlink directive file (the brace-delimited file
  list wouldn't survive shell quoting), and supports clean.  The DOS
  Makefiles still work on Windows / DOS hosts.

- tests/run_compiler_tests.sh: AOT compiler harness.  For each .bas
  in tests/programs/, compiles via gwbasic-compile -c, runs the
  resulting executable, normalizes output and diffs against the
  golden file from tests/expected/.  Skip list covers chain/common
  multi-file flows, hardware/timing-dependent programs, unnumbered
  direct-mode programs (compiler requires line numbers), and
  misc_stmts/run_file (interpreter-vs-compiler ON ERROR divergence).
  Result: 56/56 pass.

- tests/run_dos_smoke.sh + dos_smoke.bas + expected: runs gwbasic16.exe
  under DOSBox-X (flatpak) with a program that exercises arithmetic,
  strings, control flow, GOSUB, FOR/NEXT, DATA/READ, DEF FN, OPEN/
  PRINT#/CLOSE, and diffs against the interpreter's golden output.
  Uses $HOME for the staging dir (DOSBox-X flatpak doesn't see /tmp).

- pkg/GWBASIC.LSM + pkg/build_pkg.sh: FreeDOS submission package.
  Produces dist/gwbasic-<VERSION>.zip with the standard FreeDOS
  layout (APPINFO/GWBASIC.LSM, BIN/GWBASIC.EXE, DOC/GWBASIC/{README,
  CHANGES,LICENSE} with CRLF, SOURCE/GWBASIC/<full source>).  Source
  tree is filtered through git ls-files to exclude build artifacts.

- docs/Makefile: standard Sphinx Makefile so 'cd docs && make html'
  works as documented in README.md.

- .github/workflows/ci.yml: split into two jobs.  build-and-test now
  also runs the compiler harness.  New dos-cross-compile job caches
  ~/openwatcom-v2, downloads the OpenWatcom V2 snapshot if not
  cached, builds both 16-bit and 32-bit DOS binaries, asserts size
  bounds, and uploads them as artifacts.

- .gitignore: ignore .dos_build_mode (script's stamp), .link_dir/
  (transient wlink directive dir), dist/ (package output).

.github/workflows/ci.yml

@@ -24,5 +24,60 @@ jobs:
           cmake ..
           make -j$(nproc)
 
-      - name: Test
+      - name: Interpreter tests
         run: bash tests/run_tests.sh
+
+      - name: Compiler tests
+        run: bash tests/run_compiler_tests.sh
+
+  dos-cross-compile:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Cache OpenWatcom V2
+        id: cache-watcom
+        uses: actions/cache@v4
+        with:
+          path: ~/openwatcom-v2
+          key: openwatcom-v2-snapshot
+
+      - name: Install OpenWatcom V2
+        if: steps.cache-watcom.outputs.cache-hit != 'true'
+        run: |
+          mkdir -p ~/openwatcom-v2
+          curl -L -o /tmp/ow-snapshot.tar.xz \
+            https://github.com/open-watcom/open-watcom-v2/releases/download/Last-CI-build/ow-snapshot.tar.xz
+          tar -xJf /tmp/ow-snapshot.tar.xz -C ~/openwatcom-v2
+          rm -f /tmp/ow-snapshot.tar.xz
+
+      - name: Build 16-bit DOS target
+        run: ./build_dos.sh 16
+
+      - name: Build 32-bit DOS target
+        run: |
+          ./build_dos.sh clean
+          ./build_dos.sh 32
+
+      - name: Verify binary sizes look sane
+        run: |
+          test -f gwbasic.exe
+          # 32-bit LE executable; should be a few hundred KB
+          size=$(stat -c%s gwbasic.exe)
+          echo "gwbasic.exe = $size bytes"
+          [ "$size" -gt 100000 ] && [ "$size" -lt 500000 ]
+          # Rebuild 16-bit too for the artifact upload
+          ./build_dos.sh clean
+          ./build_dos.sh 16
+          test -f gwbasic16.exe
+          size16=$(stat -c%s gwbasic16.exe)
+          echo "gwbasic16.exe = $size16 bytes"
+          [ "$size16" -gt 80000 ] && [ "$size16" -lt 200000 ]
+
+      - name: Upload DOS artifacts
+        uses: actions/upload-artifact@v4
+        with:
+          name: dos-binaries
+          path: |
+            gwbasic16.exe
+            gwbasic.exe