Function env_hash 

pub fn env_hash(project_root: &Path, plugin_dirs: &[PathBuf]) -> u64

Compute environment hash from config + plugins + cha binary fingerprint.

The binary fingerprint covers both cases that make cached SourceModels stale:

• developer rebuilds cha after editing parser code,
• end user upgrades to a new cha release.

Both produce a different on-disk binary, so the binary’s modification time is sufficient. CARGO_PKG_VERSION was the old key, but it was a strict subset of this: every release-version bump necessarily writes a new binary (new mtime), and no parser change ever happens without a rebuild (new mtime). Version-only tracking missed parser-behaviour changes that shipped without a cargo xtask bump — this is what let the header-declaration parser fix silently fail against stale caches.