fix: serialize disk cache writes with stable lock

[suchapalaver]

Summary:

• protect DiskCache load/mutate/save with a stable companion lock file
• keep atomic write/rename and add a concurrent two-instance insert regression test
• replace stringly-typed blocking operation labels with typed enums

Verification:

• cargo fmt
• git diff --check
• nix shell nixpkgs#stdenv.cc -c cargo clippy --all-targets --all-features -- -D warnings
• nix shell nixpkgs#stdenv.cc -c cargo test blocks::cache::disk::tests --all-features

Closes #13