Add a cargo-checksum.json, remove its fake

See "https://wiki.debian.org/Teams/RustPackaging/Policy"

In section "Library package structure", the policy manual explains that
this file is meant to exist in the Debian package (i.e.: it's not
generated during the build) and should contain the SHA256 checksum of
the upstream crate.

The crate's internal arrangement can change depending on which version
of Cargo was used to create it. This checksum is from Rust 1.93, the
current stable at time of writing. Debian 13's Rust 1.85 produces a
different file with a different sum. I'm not sure what to do about that
right now.

debian/cargo-checksum.json

@@ -0,0 +1 @@
+{"package":"62e0ece55a4e0150ab3a3f7f5299e6eb113134975100cddde5a68dfcbb5e1a5c","files":{}}