Function register_session_read_only_root 

pub fn register_session_read_only_root(root: &Path) -> bool

Register a session-scoped read-only root (an auto-detected language cache). Returns true when newly added. Idempotent on the canonicalized path.

ponytail: process-global set, not per-session — language caches (~/go/pkg/mod, ~/.cargo/registry, …) are machine-global read-only dirs, so sharing read access across sessions grants nothing a session couldn’t already get by reading them; per-session isolation would be plumbing for no security gain. Upgrade to a session-keyed map only if writable roots ever go down this path.