Add GIMP Motif differential testing

[jserv]

Bring GIMP up under the Motif build path and validate it against system Xlib via differential screenshot test. Adds the GIMP source patch series (compat/gimp-patches), the build fragments to fetch and build it (mk/gimp-motif.mk, mk/sdl.mk), and the differential runner (scripts/ run-gimp-differential-tests.py) with its fixtures and replay.

Supporting compat fixes surfaced by the GIMP workload:

• events.c: route inline-text events through a side queue so they survive sdl2-compat's event path, with thread-local wheel deltas
• snapshot.c: write screenshots to a temp file and rename into place so readers never observe a truncated BMP
• sdl-wrapper.c: side-queue-aware SDL event wrappers, undo push-time accounting on a full ring, and a shared atomic symbol-cache macro

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Summary by cubic

Adds GIMP 0.54 (Motif) as a differential test target built against libx11-compat, plus event, snapshot, and SDL loader‑path fixes uncovered by this workload to improve stability and test reliability.

• New Features

  • Build and test GIMP 0.54 against libx11-compat + in‑tree Motif via mk/gimp-motif.mk; patch set in compat/gimp-patches (modern toolchain/LP64, updated PNG plug‑in, dropped TIFF).
  • New runner scripts/run-gimp-differential-tests.py with fixtures and a startup replay; added gimp jobs to ci.yml and differential.yml; README updated.
  • Centralized SDL detection in mk/sdl.mk; tests set runtime loader paths so sdl2-compat resolves transitive libSDL3 libraries reliably.

• Bug Fixes

  • Inline text input now rides a side queue so it survives sdl2-compat; X event accounting stays correct; wheel deltas stored thread‑locally.
  • Screenshots save to a temp file and rename() into place to avoid truncated BMP reads.
  • SDL wrapper gains side‑queue‑aware event wrappers, fixes push‑time accounting on full rings, and uses a shared atomic symbol‑cache macro.
  • Robustness tweaks: accept “schoolbook” as a font alias, include wchar.h, use XmStringUnparse for file dialogs, and default gimprc path/name to run from the build tree.

^{Written for commit 4b0e85b. Summary will update on new commits.}