chore: keep the fuzz cache and pending reproductions out of the tree

Assisted-by: GLM 5.3 Flash

.gitignore

@@ -7,6 +7,11 @@
 coverage.out
 *.test
 
+# The fuzz campaigns' cache: committed seeds live in the test file, crashes are
+# minimised into it, so everything go test -fuzz writes under testdata/fuzz is
+# transient. pending-* holds minimised reproductions awaiting their owner's fix.
+/asm/testdata/fuzz/
+
 # Crash dumps from the emulator runs
 core
 core.*