fix(packaging,host): a fresh Linux install can start, and the comments stop lying

Fallout from the documentation sweep: verifying doc claims against the code
turned up defects in the code and the shipped templates. Mostly comments that
describe behaviour we no longer have — which is how the docs went wrong in the
first place, since someone reads the comment and writes the page.

The one that mattered: a fresh deb/RPM/Arch install could not start the host at
all. The unit's `EnvironmentFile=` had no `-`, making host.env mandatory, and no
package creates it — all three ship only the templates under /usr/share and the
postinst merely prints the copy command. So `systemctl --user enable --now
punktfunk-host` died on "Failed to load environment files". Every field in
HostConfig::from_env resolves through unwrap_or/filter/None, so absent means all
defaults, exactly like a hand-run `serve`; the Nix module already wrote it as
`-${environmentFile}`. The Deck installer's own generated unit gets the same
prefix for the case where an operator later removes the file.

Shipped templates: PUNKTFUNK_SECURE_DDA is read by nothing (DDA/WGC are gone;
IDD-push is the sole Windows capture path and the secure desktop is
unconditional), so it stops being written into a fresh host.env;
PUNKTFUNK_INPUT_BACKEND offered a `uinput` value that does not exist and omitted
`kwin`, which is what a KDE session actually resolves to; PUNKTFUNK_RENDER_ADAPTER
no longer claims to pick a "Desktop-Duplication" GPU.

Comments corrected rather than deleted, since each explains a real why:
PUNKTFUNK_10BIT is default-on with explicit-off grammar, not an operator opt-in;
GNOME reaches EIS through Mutter's direct RemoteDesktop API, so it needs no portal
approval and is headless-capable; and the host does not run in session 0 — the
service is the session-0 supervisor and the host runs as SYSTEM in the interactive
console session, which is why game_term has to bind the input desktop at all.

packaging/bazzite/update-punktfunk.sh is now installed to /usr/share/punktfunk/,
so the command the docs promised exists.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/host.env.example

@@ -55,7 +55,10 @@ PUNKTFUNK_VIDEO_SOURCE=virtual
 # mid-stream. Fine for a dedicated headless appliance (punktfunk-kde-session.service, a pure
 # gamescope box) or a CI run; wrong for any box that switches sessions.
 #PUNKTFUNK_COMPOSITOR=kwin          # kwin | mutter | gamescope | wlroots | hyprland
-#PUNKTFUNK_INPUT_BACKEND=libei      # wlr | libei | gamescope | uinput (auto-routed per connect)
+#PUNKTFUNK_INPUT_BACKEND=kwin       # wlr | kwin | libei | gamescope — anything else logs "unknown
+                                    # PUNKTFUNK_INPUT_BACKEND" and auto-detects instead:
+                                    # PUNKTFUNK_COMPOSITOR above wins (except mutter), else
+                                    # XDG_CURRENT_DESKTOP — KDE -> kwin, GNOME -> libei, else wlr
 #WAYLAND_DISPLAY=wayland-kde        # headless-KDE appliance socket; retargeted per connect otherwise
 #XDG_CURRENT_DESKTOP=KDE