Wire real GBM+EGL+DrmCompositor rendering for one head (GPU Phase 2)

Extends the SRDWM_GPU=1 opt-in path (Phase 1, fa0c7f1) from a
capability probe into an actual, working GPU render pipeline for
exactly one head, per the plan this was built from
the plan file.

gpu.rs: probe now goes all the way through EGLContext, GlesRenderer,
DrmDevice (real DRM device, separate duped fd from the existing legacy
Card), and DrmOutputManager construction, returning both a GpuContext
and its DrmDeviceNotifier on success. GpuContext::initialize_output
drives one crtc/mode/connector through DrmOutputManager, storing the
resulting GpuOutput for the render loop to find.

Confirmed while reading smithay's own source directly (not assumed):
DrmDevice::new's disable_connectors parameter is not an atomic-vs-
legacy switch - DrmDevice::create_internal tries atomic capability
first and falls back to a Legacy internal variant automatically,
exposed via DrmDevice::is_atomic(), logged here rather than assumed.

platform.rs: probes at startup, calls initialize_output for the first
head only (Phase 2's deliberate scope - see the plan), registers the
DrmDeviceNotifier as its own calloop event source alongside (not
replacing) the existing legacy DRM-fd registration.

render.rs: render_udev_frame's per-head loop checks, before any of the
existing Pixman-specific element-building logic runs, whether this
head's crtc matches the GPU context's initialized output; if so,
renders a plain clear color through render_frame/queue_frame and
continues to the next head, completely bypassing the Pixman path for
that head. Every other head, and this same head whenever the GPU
context or its output is absent, is entirely unaffected.

session.rs: register_gpu_drm_notifier handles DrmEvent::VBlank by
calling frame_submitted() on the matching GpuOutput (required per
queue_frame's own doc comment, or the swapchain runs out of buffers)
and logs DrmEvent::Error without treating it as fatal.

Deliberately out of scope for this phase (documented in the plan):
window content/decorations/cursor on the GPU head (clear color only),
multi-monitor GPU rendering (one head only), and VT-switch pause/
resume for the GPU head specifically (DrmOutputManager's own pause()/
activate() calls are a different API surface from the
existing manual set_crtc+DPMS reassertion, and porting that pairing
correctly needs its own isolated verification pass).

SRDWM_GPU unset (the default) is unaffected: every step above only
runs when it's set to "1", and every failure at any step falls back
to the existing, untouched Pixman path with a logged reason, same
fallback contract Phase 1 already established.

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