deps: SDL forks carry tests, and Wayland loads on older libxkbcommon

[foxnne]

Fixes #289. Part of #280 (steps A1, A2 and A4b landed on the forks).

Moves fizzy's .sdl pin to sdl_zig fizzy-1.0.3+3.4.16-8 (SDL fizzy-3.4.16-8). That brings two things:

1. The forks carry tests now. fizzyedit/SDL has fizzy's own suites on SDL's test harness, one per patch, each failing without its patch. fizzyedit/sdl_zig runs them with zig build test-fizzy in CI: Linux on lavapipe under Xvfb and headless weston, and the hosted macOS runners, which have a Metal device.

• test: a fizzy suite for claiming a transparent GPU window SDL#1 → fizzy-3.4.16-6 (4839ede): the runner, plus patch 1's suite (transparent window claimed on Metal and Vulkan).
• build: a test-fizzy step running fizzy's SDL patch suites sdl_zig#1 → fizzy-1.0.3+3.4.16-6 (0f6a2a8): the test-fizzy step.
• test: a fizzy suite for Wayland frame insets, on the protocol trace SDL#2 → fizzy-3.4.16-8 (a518aaf), ci: run the Wayland frame-insets suite under headless weston sdl_zig#2 → fizzy-1.0.3+3.4.16-8 (a02dda8): patch 6's suite, Wayland frame insets.
  • How: it reads libwayland's request trace under headless weston, at scale 1 and 2.
  • Checks: geometry, input region, min/max sizes and published insets while floating; zero insets when maximized or fullscreen; popup anchoring.
  • Result: passes on the fork, and fails on upstream 3.4.16.
  • Problems it found in patch 6: five, listed in Testing, performance and upstreaming for fizzy's SDL patches and backend #280 (C2b).

2. SDL patch 8: Wayland loads where libxkbcommon is older than 1.10 (#289).

• The bug: built against 1.10+ headers (sdl_zig uses 1.13), SDL required xkb_keymap_mod_get_mask(). On Ubuntu 24.04 (1.6) or Debian 12 (1.5) the whole Wayland driver reported itself unavailable, and fizzy fell back to X11. Upstream main has the same problem.
• The fix: the symbol is now optional at runtime, and the masks fall back to xkb_keymap_mod_get_index(), the path SDL already takes when built against older headers. For standard keymaps both paths give the same masks.
• The proof:
  • red on ubuntu:24.04 without the patch ("wayland not available");
  • green on 24.04 and 26.04 with it, where a German keymap's Shift, Caps Lock and AltGr levels come out right.
• The PRs: wayland: xkb_keymap_mod_get_mask() optional at runtime, with a suite SDL#3 → fizzy-3.4.16-7 (36dda37); test: run the Wayland keyboard suite on Ubuntu 24.04 and 26.04 sdl_zig#3 → fizzy-1.0.3+3.4.16-7 (264af2a).

docs/DEPENDENCIES.md records patch 8, the three suites, sdl_zig's step and CI jobs, all six tags, and each patch's check at a rebase.

Verified

On macOS (arm64), in a fresh workspace on main:

Gate	Result
zig build	43/43 steps
zig build test	75/75 steps
zig build test-integration	33/33 steps, 347/347 tests
zig build check-web	16/16 steps
zig build test-sdk-version	12/12 steps
zig build -Dtarget=x86_64-linux-gnu	54/54 steps (builds the patched Wayland file)

Not verified: a fizzy binary running on a real Ubuntu 24.04 Wayland desktop. The evidence is the fork suite in CI containers, built from the same sdl_zig. The SDK fingerprint doesn't move, since plugins never link SDL.

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