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/mdlp-image.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Builds the mdlp-image (mdlp-image/Dockerfile) locally, tagged with
+# mdlp-image/VERSION and :latest. Does not push - test it with
+# ./mdlp-run-image.sh first, then publish with ./mdlp-upload-image.sh.
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")"
+ROOT="$(pwd)"
+
+fail() { echo "PREFLIGHT FAIL: $*" >&2; exit 1; }
+
+command -v docker >/dev/null 2>&1 \
+    || fail "docker not found in PATH"
+
+IMAGE="${MDLP_REGISTRY_IMAGE:-questshock/mdlp}"
+if [ -f "$ROOT/../registry.env" ]; then
+    # shellcheck disable=SC1091
+    source "$ROOT/../registry.env"
+    IMAGE="${MDLP_REGISTRY_IMAGE:-$IMAGE}"
+fi
+
+VERSION="$(<"$ROOT/mdlp-image/VERSION")"
+[ -n "$VERSION" ] || fail "mdlp-image/VERSION is empty"
+
+echo "== Building $IMAGE:$VERSION =="
+docker build \
+    -f "$ROOT/mdlp-image/Dockerfile" \
+    -t "$IMAGE:$VERSION" \
+    -t "$IMAGE:latest" \
+    "$ROOT"
+
+echo "== Done =="
+echo "Image: $IMAGE:$VERSION (and :latest)"
+echo "Test it with ./mdlp-run-image.sh, then publish with ./mdlp-upload-image.sh"