feat(x11): MIT-SHM 1.2 fd-passing present fast path (measured win, PutImage fallback)

[tannevaled]

Task 2 — MIT-SHM present for X11 (measure-first)

Plain PutImage ships every pixel of an update through the X socket. This adds
the MIT-SHM 1.2 path: the client writes pixels into a shared-memory segment
both peers map and sends a tiny ShmPutImage request instead. The segment
descriptor is passed via AttachFd over SCM_RIGHTS, reusing the same
sovereign fd-passing machinery as the Wayland backend (memfd-style
shm_open+ftruncate+mmap; no SysV shmget).

Measured on the Xvfb runner (800×600, depth 24, 300 iters)

Reproduced by TestLiveX11SHMMeasure, uploaded as shm-measure.txt:

update	path	bytes/present	ms/present
full window	PutImage	1,920,192	2.96
full window	ShmPutImage	40	1.76
small damage (32×32)	PutImage	4,120	0.044
small damage (32×32)	ShmPutImage	40	0.042

Full-window: ~48,000× fewer bytes over the socket and ~40 % lower latency.
This backend presents the whole framebuffer after input, so the full-window
win dominates → SHIP. Plain PutImage stays as the fallback whenever the
server lacks the extension, lacks fd-passing, or is < 1.2.

What changed

• internal/x11: QueryExtension; MIT-SHM QueryVersion/AttachFd/Detach/
  ShmPutImage; Presenter.EncodeRectInto/SegmentSize; NewSegment
  (shared mmap); WrapUnix transport adapter + exported FDSender.
  100 %-covered — both endian paths, the real SCM_RIGHTS fd path over a
  socketpair, and every syscall failure branch.
• window: Open() auto-enables SHM when available; present() mirrors the
  damage rect into the segment and blits with ShmPutImage; the segment grows
  on resize; Close detaches. Silent fallback to PutImage on any failure.
• Live proof: TestLiveX11 now draws through ShmPutImage (asserted active by
  TestLiveX11SHMActive), so its captured-pixel assertions prove SHM present
  renders correctly — nothing faked.
• CI: live X11 lane renamed (Xvfb, +MIT-SHM); uploads shm-measure.txt.

internal/x11 and internal/wayland both at 100.0 % coverage under -race.

🤖 Generated with Claude Code