fix(track-a): restore canonical Qt XCB GL integration selection

[blakinio]

Track A hosted-only graphics integration repair

Task: OTC-20260816-track-a-canonical-graphics-integration-fix
Base: exact trusted main@a482bba877c881d31ae903a6f8acad24debfb5c5.
Execution: GitHub-hosted only, runtime_access: none, no physical E2E in this PR.

Terminal discriminator #398 directly proved the exact client remained alive for 35 seconds with zero visible X11 windows and logged Qt failures stating that neither GLX nor EGL was enabled. Qt 6.9.3 primary source (qt/qtbase, QXcbConnection::glIntegration) proves the trusted worker's explicit QT_XCB_GL_INTEGRATION=none clears both default XCB GL integration candidates (xcb_glx, xcb_egl).

This minimal fix:

• removes only QT_XCB_GL_INTEGRATION=none from the canonical client environment;
• preserves QT_QUICK_BACKEND=software;
• adds non-secret QSG_INFO=1 graphics initialization diagnostics;
• does not force QSG_RHI_BACKEND or a particular GLX/EGL implementation;
• adds a hosted source-contract test pinning those invariants.

No exact-client fence, WARP/X11/VNC, lease, registration, Gate B, rollback, credential/login, Track B or runtime-ownership contract is weakened. This PR does not claim GLX/EGL availability on Synology or visible-window success; those require a later fresh physical RUNTIME admission after this fix reaches trusted main.

[blakinio]

COORD outcome: ACCEPT on exact head 74d5c09f53db42e69c703b06d06c4327a4913cec.

Material findings open: 0.

Evidence: Qt 6.9.3 primary source proves QT_XCB_GL_INTEGRATION=none clears the XCB GL integration candidate list; terminal runtime discriminator #398 directly observed matching GLX/EGL-disabled Qt failures with the exact client alive and zero visible windows. The fix is intentionally minimal: remove only that explicit disablement, preserve QT_QUICK_BACKEND=software, add non-secret QSG_INFO=1, and do not force QSG_RHI_BACKEND.

Hosted validator 31959453898 / 95195086514 passed session 11, transition 9, guard 3, lease 14, bash syntax, and graphics environment contract with runtime_access:none. Temporary validator removed. Exact-head Track A governance 31959541472 and repository CI 31959541458 are SUCCESS; review threads 0. Physical visible-window success remains unclaimed and belongs to a later fresh RUNTIME admission.

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