Vendor the Shockolate engine and build it via a Docker image with every

dependency (SDL2, SDL2_mixer, fluidsynth-lite, a MIDI soundfont)
prebuilt, so compiling the engine needs no network access - just the
image and the engine source. Add res/assets/extract_assets.sh to pull
the game's data files out of a purchased GOG installer, and a Makefile
that assembles a runnable dist/ from the two.

Also add `make package`, which builds a redistributable tarball that
omits the proprietary game assets (shipping res/GET_ASSETS.txt instead)
plus license information for both the MIT tooling and the GPLv3 engine,
and a Gitea Actions workflow that builds and publishes it to
dl.ladkau.de on every push.

res/assets/GET_ASSETS.txt

@@ -0,0 +1,16 @@
+This package does not include System Shock's game data - it's
+copyrighted, proprietary content that can't be redistributed. To play,
+you need a copy of System Shock: Enhanced Edition (e.g. from gog.com).
+
+From your Enhanced Edition install, you need its classic-game data and
+sound files - the res/pc/hd/data and res/pc/cdrom/data trees merged
+together (res/pc/hd's copies win the couple of filenames present in
+both: intro.res, objprop.dat), and the res/pc/hd/sound tree, packed
+inside the install's sshock.kpf (a zip file).
+
+Copy that merged data into place, alongside this file:
+  res/data/   <- res/pc/hd/data + res/pc/cdrom/data, merged
+  res/sound/  <- res/pc/hd/sound
+
+Once res/data/ and res/sound/ exist next to this file, run ./run.sh from
+the root of this package to play.