docs(steamos): reframe Steam Deck host page to SteamOS

- Rename steam-deck-host.md → steamos-host.md (nav + install table updated).
- Lead with the rationale: SteamOS host support targets the upcoming Steam
  Machine; the Steam Deck is the SteamOS device validated against today.
- Soften the WiFi note: ~250 Mbps was our testing on one device/network,
  not a universal ceiling — other SteamOS hardware/drivers/bands may do more.
- Generalize Deck-specific language to SteamOS devices throughout.
- Document --no-gamestream (secure native-only) + GameStream-compat caveat.
- decky README: drop stale `serve --native` (now just `serve`).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/decky/README.md

@@ -81,7 +81,7 @@ argv and a clear `client-not-found` error surface to the UI. The child PID is tr
   installed and runnable on the Deck — via `.deb`/RPM/flatpak, or symlinked into
   `~/.local/bin`.
 - **avahi** (`avahi-daemon` + `avahi-browse`) for discovery — present on SteamOS/Bazzite.
-- A punktfunk/1 host on the LAN (`punktfunk-host serve --native` or `punktfunk1-host`).
+- A punktfunk/1 host on the LAN (`punktfunk-host serve` or `punktfunk1-host`).
 
 ## Build