chore: release 0.15.4

[suchapalaver]

Summary:

• bump crate version to 0.15.4
• add changelog entries for Price scans can cache incomplete log coverage as authoritative #12 and DiskCache needs a stable lock for cross-instance read-modify-write #13 patch fixes

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 --all-features
• nix shell nixpkgs#stdenv.cc -c cargo test
• nix shell nixpkgs#stdenv.cc -c cargo test --no-default-features
• nix shell nixpkgs#reuse -c reuse lint
• nix shell nixpkgs#stdenv.cc -c cargo publish --dry-run
• nix shell nixpkgs#stdenv.cc nixpkgs#cargo-semver-checks -c cargo semver-checks check-release