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>

tests/core_tests.cpp

@@ -2115,6 +2115,8 @@ int main() {
               {"gate_scratches", 0L}},
              {{"gate_weave", 0L}, {"gate_grain", 0.0}, {"gate_flicker", 0.0}, {"gate_dust", 0L},
               {"gate_scratches", 0L}}},
+            {"datamosh", {{"mosh_rate", 0.4}, {"mosh_block", 8L}, {"mosh_bleed", true}}, {},
+             {{"mosh_rate", 0.0}}},
         };
         for (const auto& [name, lively, smooth, off] : moving) {
             const auto effect = effectByName(name);