docs(linux): park Slice L at L1 checkpoint

docs/research/2026-07-27-slice-l1-graphical-capability-checkpoint.md

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+# Slice L1 graphical capability checkpoint
+
+**Date:** 2026-07-27
+**Production commit:** `11501d52cad942850d88f83490710faf1d5cdf28`
+**Status:** implementation complete; further Linux graphical work deferred by
+the user on 2026-07-27. The supported physical-Linux driver matrix remains the
+first gate when Slice L resumes.
+
+## Scope
+
+This checkpoint establishes the Linux/Windows graphical startup oracle before
+renderer construction. It does not add a renderer fallback and does not change
+gameplay or visual behavior.
+
+`GraphicalHostPlatformServices` now owns an immutable GLFW backend selection.
+`ACDREAM_DISPLAY_PROTOCOL=auto|x11|wayland` is consumed once before
+`Window.Create`. The platform owner configures the exact
+`GlfwProvider.UninitializedGLFW` instance used by Silk's window backend.
+
+Silk 2.23 asks for `libglfw.so.3.3`, while its packaged multi-backend GLFW 3.4
+asset is `libglfw.so.3`. The default native search order previously selected
+Ubuntu's machine-global GLFW 3.3 before reaching the packaged 3.4 library.
+Startup now prioritizes the application directory inside Silk's path resolver,
+so X11/Wayland selection and package provenance are deterministic.
+
+Before any renderer object is created, the client records:
+
+- OS, RID, requested and active display protocol, GLFW backend/version;
+- GL vendor, renderer, version, GLSL, context flags/profile, limits, and every
+  extension;
+- default framebuffer color/depth/stencil/MSAA/sRGB facts;
+- window, monitor, refresh, framebuffer/logical size, and input-device facts;
+- audio request/availability/playback/disposal state;
+- window/GL/input/audio ownership and terminal shutdown state.
+
+Advertisement is not treated as proof. A balanced active probe executes:
+
+- bindless texture handle, resident, and non-resident calls;
+- a shader using `gl_DrawIDARB`/`gl_BaseInstanceARB` plus MDI;
+- SSBO allocation and binding;
+- a timer query;
+- an sRGB color plus depth/stencil framebuffer;
+- persistent coherent mapping whenever buffer storage is advertised.
+
+Unsupported hardware receives one actionable report and exit code 4. The
+Studio smoke defers native close until after `RenderStack.EndFrame`, so both
+successful and rejected startup release every owned context resource before
+the final report is atomically rewritten.
+
+## Positive supported gate
+
+Windows 11 Pro build 26200, AMD Radeon RX 9070 XT, driver
+`32.0.31021.5001`:
+
+- GLFW `3.4.0 Win32 WGL`;
+- OpenGL `4.3.0 Core Profile Context 26.6.4.260624`;
+- 240 Hz monitor, 1280 x 720 logical/framebuffer smoke;
+- bindless, shader draw parameters, MDI, SSBO, timer query, sRGB
+  depth/stencil, and persistent coherent storage all actively pass;
+- OpenAL Soft default device opens, accepts the 50 ms smoke wave, and disposes;
+- the 1280 x 720 retained-UI mockup screenshot is visually intact;
+- process exits 0 with zero owned window, GL, input, and audio resources and
+  `ShutdownComplete=true`.
+
+## Linux backend and negative-capability gates
+
+Package: exact framework-dependent `linux-x64` publish of
+`11501d52cad942850d88f83490710faf1d5cdf28`.
+
+Host: Ubuntu 24.04.3 LTS under WSL2 kernel
+`6.6.87.2-microsoft-standard-WSL2`, WSLg X11 and Wayland endpoints.
+
+Both explicit backends create an OpenGL 4.5 core context, expose one keyboard
+and one mouse, capture their framebuffer/monitor facts, and identify the
+packaged multi-backend GLFW 3.4:
+
+| Protocol | Monitor | Renderer | Result |
+|---|---|---|---|
+| X11 | `XWAYLAND0`, 60 Hz | Mesa llvmpipe 25.2.8 | exit 4: bindless absent |
+| Wayland | `weston rdp`, 60 Hz | Mesa llvmpipe 25.2.8 | exit 4: bindless absent |
+| X11 + `GALLIUM_DRIVER=d3d12` | `XWAYLAND0`, 60 Hz | D3D12 (AMD Radeon RX 9070 XT), GL 4.6 | exit 4: bindless absent |
+
+Each final report names only `GL_ARB_bindless_texture` as the mandatory
+missing feature, preserves the complete capability record, and reports zero
+window/GL/input/audio ownership with `ShutdownComplete=true`. This is the
+intended mandatory-modern-renderer failure, not an accepted Linux driver row.
+
+The Ubuntu graphical CI job now builds/publishes the App, runs the focused
+platform/capability tests, checks the native package closure, and verifies the
+same actionable X11 software-driver rejection under Xvfb.
+
+## Automated gates
+
+- graphical backend/capability/Studio focused tests: 29/29;
+- App Release tests: 3,763 passed / 3 skipped;
+- complete Release solution: 8,826 passed / 5 skipped;
+- Release solution build: zero warnings and zero errors;
+- Windows supported smoke: exit 0, screenshot present, all function probes
+  pass, audio playback submitted, all terminal ledgers zero;
+- WSLg X11 and Wayland negative gates: exit 4, exact bindless diagnostic,
+  all terminal ledgers zero;
+- concurrent two-account Linux headless process remained 2/2 in-world with
+  zero faults/reconnects/resource-envelope violations throughout the gates.
+
+## Deferred L1 acceptance
+
+Do not claim Linux graphical support from WSLg. L1 still requires at least one
+physical supported Linux row and the committed driver matrix:
+
+- AMD Mesa/RadeonSI X11 and Wayland;
+- NVIDIA proprietary X11 and supported Wayland;
+- Intel only when every mandatory capability is present.
+
+On each supported row, rerun the packaged screenshot/input/audio/teardown
+smoke. Only then close L1 and begin the connected L2 route. This work and
+Slices L2–L6 were explicitly parked by the user on 2026-07-27.
+
+## Rollback
+
+```text
+git revert 11501d52cad942850d88f83490710faf1d5cdf28
+```