All 3605 .dds under gfx/event_pictures/ are now 512x256 + DXT5 (BC3)
+ 10-level mip chain. Non-2:1 sources (e.g. 512x291) are center-cropped
to 2:1; everything is resized with Lanczos.

Total size: 1194.7 MB -> 601.4 MB (-49.7%, 593.3 MB saved).

Format rationale: vanilla EU4 has 416 DXT5 and 0 BC7 instances in its
8381 DDS files; DXT5 is the proven format for the Clausewitz engine.
DXT1 was considered for size, but a full survey showed 24% of files
(869) have non-trivial alpha distribution (true transparency, not
artifacts), so DXT5 is required to preserve them.

Re-uses tools/bin/texconv.exe shared with the loading-screen pipeline.

New tool: tools/convert_eventpicture_dds.py
-j N parallel workers (default 4)
--dry-run report without writing

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>