Merge pop-os/cosmic-epoch: become the meta-repo, with two submodules swapped

HyprCosmic started as a repository beside COSMIC holding a config compiler and a
pile of theme assets, with the two modified components hanging off it. That had
the relationship backwards. COSMIC already has a meta-repo whose whole job is to
name every component and build the desktop from them, so the honest shape for a
fork is to be that meta-repo with the components we changed pointing at our
copies -- not a separate tree that assumes the rest of COSMIC arrived some other
way.

So this merges cosmic-epoch in and repoints exactly two of its 29 submodules:

    cosmic-comp     -> outbackdingo/hyprcosmic-comp
    cosmic-session  -> outbackdingo/hyprcosmic-session

The other 27 stay on pop-os. Nothing about them needs to change, and pinning
them to copies we do not maintain would be a promise to keep 27 forks current.

`just build` and `just install` now build and install the whole desktop with our
compositor and session in it, which is what "fork COSMIC" ought to mean. The two
forks still install into /usr/libexec/hyprcosmic rather than over /usr/bin, so a
system that already has COSMIC from its distribution keeps that session on the
greeter's menu alongside this one.

Conflict resolutions worth stating:

  .github/workflows/ci.yml   upstream's kept as-is. It builds the entire desktop
                             on Arch through `just sysext`, which is precisely
                             the check a meta-repo wants and is not made less
                             useful by forking. Our per-distribution workflow
                             moved to hyprcosmic.yml beside it, the same way it
                             did in the compositor fork.

  .gitmodules                upstream's 29 entries, then two URLs rewritten.

  cosmic-comp, cosmic-session
                             ours. Git reports "no merge base" because they are
                             unrelated to the pop-os commits recorded here,
                             which is expected: they are different repositories,
                             not newer commits of the same one.

  .gitignore                 both sides, plus a note that the two forks are
                             deliberately no longer ignored.

docs/DEBUGGING.md

@@ -0,0 +1,41 @@
+Debugging COSMIC
+================
+
+An assortment of useful tools and settings for debugging COSMIC.
+
+## Logs
+Cosmic-comp, cosmic-panel, and other components log to `stderr`, as well as journald (if present).
+
+## Wayland and X11 Protocols
+
+Run clients with `WAYLAND_DEBUG=1` to see what wayland calls are made. `xtrace -n <command>` can be used to see what X calls an X11 client is making.
+
+## XWayland
+
+A tool like `xprop` can be used to determine if an application is running in XWayland. If `xprop` is unable to select a window, it is a native Wayland window.
+
+## Performance
+
+`sudo perf top` can be used to see what functions, across all processes, are using the most CPU time. The `-p` argument can be used to restrict this to a single process. The output is more useful for executables with debug symbols.
+
+`cosmic-comp` integrates Tracy for profiling. It can be built with `cargo build --features profile-with-tracy --profile fastdebug`, then the Tracy client can connect from the same system or a different one
+
+## Graphics drivers
+`eglinfo` and `vulkaninfo` indicate what graphics cards and drivers are in use for hardware-accelerated rendering.
+
+On systems with NVIDIA graphics, `nvidia-smi` has information about the GPU and driver.
+
+## DRM
+The Linux DRM subsystem handles graphics cards and display controllers.
+
+`drm_info` prints information about the outputs, planes, etc. associated with each GPU and their properties. The output of this command can be useful for understanding display related issues.
+
+https://gitlab.freedesktop.org/wlroots/wlroots/-/wikis/DRM-Debugging documents how to configure DRM to enable additional logging, which can be useful for understanding some DRM driver issues.
+
+## Frozen desktop
+
+If the desktop is frozen and `ctrl+alt+f*` don't work to change to a TTY, [the magic SysRq key](https://www.kernel.org/doc/html/latest/admin-guide/sysrq.html) with `r` can be used to switch input into raw mode, so the kernel will handle the tty switch key binding. `/etc/sysctl.conf` or `/etc/sysctl.d` may need to be edited first to set `kernel.sysrq=1` or another value that allows this command.
+
+It is also possible to connect over `ssh` from another computer.
+
+Then `pidof cosmic-comp` can be used to get the PID of the compositor. Then it is possible to investigate further with tools like `gdb`, using `sudo gdb` and then `attach <pid>`.