linux: the desktop's blur behind the window, behind FIZZY_BLUR_BEHIND=1

[foxnne]

Part of #226 (docs/NATIVE_WINDOWS_PLAN.md, "Linux — the compositor's blur"); step 4 of docs/WINDOWS_LINUX_GLASS_PLAN.md (#236).

What changes

With FIZZY_BLUR_BEHIND=1, on a Wayland compositor that blurs, the window's base goes translucent at the window opacity over the desktop's blur, as it already does over macOS's vibrancy and Windows' Acrylic. Elsewhere, and without the switch, nothing changes: the window stays opaque.

• platform/wayland_blur.zig: binds ext-background-effect-v1 (GNOME 51, Plasma 6.7, niri) or KDE's org_kde_kwin_blur.
  • It uses SDL's own libwayland-client (RTLD_NOLOAD), with both protocols declared by hand, on an event queue of its own.
  • The region is sent only when it changes, and it lands with the commit the frame's present makes.
  • "No blur" is an empty region on the ext protocol, and the blur object unset and released on KDE's protocol (KWin blurs the whole window behind an empty one).
  • The first probe logs what it found.
• platform/blur_region.zig (std-only, unit tested): steps the rounded frame into rectangles.
• linux_titlebar.blurBehind and Editor.tick: the blur covers the frame inside the shadow's margin while the window is translucent, and is square when maximized.

Off by default until someone sees it on a desktop that blurs.

SDK impact

• None

Verified

• Linux, Ubuntu 26.04 / GNOME 50 (aarch64 UTM VM, native Wayland, FIZZY_BLUR_BEHIND=1 WAYLAND_DEBUG=client). GNOME 50 has neither protocol, so this covers the "absent" path only:
  • one get_registry on fizzy's own unnamed queue;
  • the log line ext-background-effect-v1 absent, org_kde_kwin_blur absent;
  • no wl_display.error;
  • fizzy alive after 12 s, window opaque.
• Protocol details:
  • opcodes checked against wayland.app (ext-background-effect-v1) and plasma-wayland-protocols' blur.xml;
  • KWin's empty-region-means-whole-window rule read in src/plugins/blur/blur.cpp on Plasma/6.6. Master only takes a non-empty region.
• A stub compositor speaking both protocols, in the cloud session that wrote this: the region applied on commit, unchanged frames sent nothing, cleared when opaque, KDE's unset, nothing asked when capabilities = 0.
• fizzy-blur-region-tests (5 tests).
• Gates: zig build, test, test-integration, check-web, test-sdk-version, and the Windows and Linux cross-builds.
• Not seen where the blur would actually show: GNOME 51, Plasma 6.7+, or Plasma ≤6.6 (KDE's protocol and the "off" path). Please try one of those before this goes default-on:
  • the blur sits inside the frame, not in the shadow margin;
  • the corners line up;
  • maximized is square and full;
  • opacity 1 gives no blur.
• macOS / Windows / Web: no change there (the code is comptime-gated to Linux; it builds on all of them).

Relies on the envSwitch fix (#235, merged): before it, FIZZY_BLUR_BEHIND read as FIZZY_POPOUT.

Follow-ups

• Default-on once seen on GNOME 51 or Plasma.
• Wayland popups (menus and dialogs as xdg_popups) are the other half of step 4, not built.

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