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.

modding/Makefile

@@ -1,16 +1,24 @@
 # Builds the modding subproject - see README.md.
 #
 # Targets:
-#   all (default)   - alias for `hacked`.
+#   all (default)   - `hacked` and `mods`.
 #   build-image      - builds the hacked-build-image Docker image
 #                       (./hacked-build-image.sh). Only needed once, or after
 #                       hacked-build-image/ or hacked/go.mod|go.sum changes.
 #   hacked           - compiles hacked/ for Linux and Windows via that image,
 #                       offline. Output lands in dist/.
+#   mdlp-image       - builds the mdlp-image Docker image (./mdlp-image.sh).
+#                       Only needed once, or after mdlp-image/ or
+#                       mdlp/go.mod|go.sum changes.
+#   mods             - builds every mod under mods/ via that image, offline.
+#                       Set MODS="name1 name2" to build only those. Output
+#                       lands in dist/mods/.
 #   clean            - removes dist/.
-.PHONY: all build-image hacked clean
+.PHONY: all build-image hacked mdlp-image mods clean
 
-all: hacked
+export MODS
+
+all: hacked mods
 
 build-image:
 	./hacked-build-image.sh
@@ -26,5 +34,16 @@ hacked:
 		./hacked-build-run-image.sh; \
 	fi
 
+mdlp-image:
+	./mdlp-image.sh
+
+# Same already-in-the-image shortcut as `hacked` above, for MDLP_IMAGE.
+mods:
+	@if [ -n "$$MDLP_IMAGE" ]; then \
+		bash mdlp-image/process-mods.sh; \
+	else \
+		./mdlp-run-image.sh; \
+	fi
+
 clean:
 	rm -rf dist