Add a first-party MDL mod-build system and reorganize modding/ under src/

modding/mods/ holds message edits as plain JSON (type, index, language,
plus only the changed fields - a long body can live in a sibling .txt to
avoid escaping in JSON), so mod authoring is reviewable in git instead of
living only in hacked's GUI. mdlp (modding/src/mdlp/, its own Docker image
mdlp-image/) turns them into the same mod folder/zip hacked's own Save
produces, reusing hacked's ss1/edit, ss1/world and ss1/content/text
packages via a go.mod replace - no GUI/GLFW/GTK needed. `make mods`
builds every mod into dist/mods/<name>/, scoped per-mod so it never
touches hacked's own build output; `make` (no target) now builds both.

Fixes a bug this surfaced: hacked's FileLocation.AbsolutePathFrom
silently writes into the current directory instead of the given output
folder when passed a relative path (the GUI never hits this since its
folder picker is always absolute) - mdlp now resolves its own paths
up front.

res/run-mods.sh/.bat are new companion launchers to run.sh/.bat: they
pass every dist/mods/*/mod folder to the engine as a separate absolute
argument, since ProcessModDirectory() only scans one directory level
deep. Wired into `make dist`/`dist-win` only, not the release archives.

modding/hacked/, its patches, sqweek-dialog/, its patches, and the new
mdlp/ move under modding/src/, separating source from the surrounding
Docker/script scaffolding; all path references updated to match.

NOTICE.txt gains a section for hacked's distributed build, listing hacked
(New BSD), sqweek/dialog (ISC - corrected from a wrong "New BSD" note),
and hacked's other direct Go dependencies.

NOTICE.txt

@@ -35,3 +35,17 @@ cross-build") additionally bundles:
   statically into systemshock.exe instead of shipped as a DLL, under the
   GCC Runtime Library Exception - this doesn't subject the rest of the
   binary to the GPL.)
+
+The modding tools' `hacked` build (see modding/README.md; output under
+modding/dist/*.tgz and *.zip) additionally bundles:
+
+- InkyBlackness HackEd (https://github.com/inkyblackness/hacked), vendored
+  unmodified as modding/src/hacked/. It is New BSD-licensed.
+- github.com/sqweek/dialog, vendored and patched (see
+  modding/src/sqweek-dialog-patches/) as modding/src/sqweek-dialog/. It is
+  ISC-licensed.
+- hacked's further direct Go dependencies (fetched at build time, not
+  vendored in this repository): github.com/inkyblackness/imgui-go (New BSD);
+  github.com/go-gl/glfw, github.com/go-gl/mathgl, and golang.org/x/image
+  (BSD 3-Clause); github.com/go-gl/gl, github.com/asticode/go-astisub, and
+  github.com/lucasb-eyer/go-colorful (MIT).