[jwinarske]

This moves the parity baseline from drm-cxx 2b260d0 to 5e7047b, which is v5.0.0 plus the merge of drm-cxx#386.

All 12 upstream commits to src/ in 2b260d0..5e7047b are now in drmkit:

• Earlier PRs: Zero SDE-private plane state another client left (P-99) #116, Clamp captured color to its alpha, as the reference now does (P-96) #118, Pause and resume the color pipeline, as the reference now does (P-82) #119 and Tell MediaTek and VeriSilicon modifiers apart, as the reference does (P-102) #123–Stack the cursor's plane at the top of what the CRTC blends (P-106) #127, plus fixes drmkit made first (#367–#371).
• This PR: the two commits in drm-cxx#386, which is upstream's fix for #384.
  • Its cursor zpos rule is identical to Stack the cursor's plane at the top of what the CRTC blends (P-106) #127's.
  • Its scene clip uses the same math as Clip a layer starting off the top or left of the CRTC (P-100) #120, except for which formats it skips. Upstream skips a list of chroma-subsampled formats. drmkit only admitted formats with one bits-per-pixel value, so 4:4:4 YUV such as NV24 kept a negative position.
  • clips_cleanly now uses upstream's list, and clip_leading gets its guard against a source extent past 32 bits (P-107).
  • Upstream's seven PlaneClip cases are ported as plane_clip_*. The NV24 case fails against the old rule.

Also here, ahead of its upstream merge: the cursor's overlay pick skips multirect virtual planes, as drm-cxx#387 does. The drm-cxx session hit one on the SA8155P ("r1 only virt plane"). Folded in here to save a separate CI run (P-108). a_multirect_virtual_plane_is_not_picked_for_the_cursor fails without it.

Baseline pins moved:

• the CI drift lane's checkout (full SHA);
• TEST_PARITY.md tracked tree;
• INVARIANTS.md;
• both READMEs;
• the workspace manifest;
• drmkit::UPSTREAM_BASELINE.

TEST_PARITY.md gains rows for upstream's three new test files:

• test_plane_clip.cpp;
• test_cursor_overlay_clip_vkms.cpp, which notes the agreed P-101 divergence;
• test_dumb_buffer_vkms.cpp.

All three are ported. The README status counts follow: 111 files, 78 ported, 14 partial, 3 not applicable, 16 pending.

Checks: cargo xtask check against 5e7047b passes (111 files, all mapped), as do clippy over the workspace with all features, workspace tests, the scene and cursor tests on vkms, and the rustdoc matrix.