Fix hacked GTK3/GLFW crash by vendoring a patched sqweek/dialog

The built hacked binary segfaults on Linux the moment a native GTK3
dialog (file/folder browse) opens while a GLFW/OpenGL context is live:
sqweek/dialog's Linux backend never calls XInitThreads() before
gtk_init(), and GLFW's own X11 backend installs a process-wide Xlib
error handler that collides with GTK's.

Vendor a pinned copy of github.com/sqweek/dialog (New BSD) into
modding/sqweek-dialog/ so it can be patched like every other vendored
source here: sqweek-dialog-patches/01-xinitthreads.patch adds the
missing XInitThreads() call, and hacked-patches/01-use-patched-dialog.patch
adds a go.mod replace directive pointing hacked's own sqweek/dialog
dependency at that local copy. build-hacked.sh now stages both hacked/
and sqweek-dialog/ into a scratch modding/build/ directory and applies
their patch series before compiling, rather than building hacked/ in
place; .gitignore picks up that new scratch directory.

Also expand modding/README.md: number its existing sections and add a
worked "Example: Editing a message" walkthrough covering Static World
Data, the Messages window's clipboard-only text editing, saving the
Mod Location, and the absolute-path requirement for run.sh's mod-folder
argument.

modding/hacked-patches/01-use-patched-dialog.patch

@@ -0,0 +1,13 @@
+--- a/go.mod
++++ b/go.mod
+@@ -2,6 +2,10 @@
+
+ go 1.25.0
+
++// Local, patched copy fixing a GTK3/GLFW X11 crash on Linux - see
++// ../sqweek-dialog-patches/ and README.md.
++replace github.com/sqweek/dialog => ../sqweek-dialog
++
+ require (
+ 	github.com/asticode/go-astisub v0.12.1
+ 	github.com/go-gl/gl v0.0.0-20210315015930-ae072cafe09d