Docs: feature-gap survey vs full DEs, and titlebar/decoration research

Two research-only entries, no code changes:

- docs/FEATURE_GAP.md gains a "vs. full desktop environments" section
  (KDE/GNOME/XFCE/macOS/Windows), requested directly and distinct from
  the file's existing tiling-WM (niri/sway/Hyprland) comparison. Verified
  rather than assumed: real app-to-app clipboard already works
  (delegate_data_device!), drag-and-drop between real windows already
  works; genuine gaps are compositor-level blur-behind, the
  already-tracked fractional-scale wl_pointer bug's real-world cost, and
  no PipeWire screencasting - with an explicit line drawn between
  srdwm's own scope and AGS's (notifications, applets, alt-tab UI,
  screenshot tooling are shell concerns, not compositor gaps).

- docs/TODO.md: researched "different titlebars, non-traffic-light,
  right side, especially firefox/chrome" and found the requested system
  already exists and is already documented (button_style, button_side/
  order, glyph-always, and a real, already-correct xdg-decoration
  negotiation with Firefox's own specific behavior already documented).
  One real, unverified gap found via actual web research into
  Chromium's own Wayland decoration history: likely_draws_own_titlebar
  only matches org.gnome.* today, and Chromium's xdg-decoration support
  has a documented history of inconsistency vs Firefox/GTK. Deliberately
  not blind-fixed - forcing decorated=false for Chrome would be worse
  than doing nothing if it already negotiates correctly; needs a live
  screenshot check with a real Chrome/Chromium install first.

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