Add Datamosh, a video effect whose Corrupt blocks break and heal over the clip

Corrupt's block damage moves to effects/corrupt_blocks.hpp so both share it;
Corrupt renders exactly as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -31,6 +31,11 @@ commit that made it. The interface shows this file under Changelog in its menu.
   the gate, flicker, dust and scratches. On the command line it is
   `-e filmgate` with `--gate-grain`, `--gate-weave`, `--gate-flicker`,
   `--gate-dust` and `--gate-scratches`.
+- Datamosh, a video effect: blocks of the picture break as Corrupt breaks
+  them, each for a stretch of the clip before it heals, and a displaced
+  block's picture smears along while it shows. On the command line it is
+  `-e datamosh` with `--mosh-rate`, `--mosh-hold`, `--mosh-drift`,
+  `--mosh-speed` and the rest.
 - A Quality choice under Video on Setup: Small file, Standard or High, for
   how the saved video is encoded. It is kept in the project, and on the
   command line it is `--video-quality small|standard|high`. Standard writes