[Upstream nexum-runtime] Cross-module local-store namespace collision on duplicate module names

[lgahdl]

Tracking issue — the bug and its fix live upstream in nullislabs/nexum-runtime, which shepherd-engine depends on as a pinned git dependency.

Upstream issue: nullislabs/nexum-runtime#61

Found during an internal red-team review (malicious module developer persona): two modules configured with the same [module].name in shepherd's own engine.toml would silently share one local-store namespace, since nexum-runtime performs no uniqueness check across configured module names at boot.

This issue tracks: re-verifying the fix once nexum-runtime ships it, and bumping shepherd's pinned dependency revision.