security(edge): bind private Cargo fallback URL to exact crate resource #115

assertProviderDestination() currently gives GitHub and npm path-level resource confinement, but the cargo-registry case only checks url.origin === grant.origin.

That leaves a confused-deputy gap: a capability granting crate A on a private Cargo registry can plan a request to another path/crate on the same origin if the broker credential is broader than one crate.

Do not guess one private-registry URL layout. First define/version the supported private Cargo request forms, then:

• bind every allowed URL form to the exact grant.resource crate name;
• reject path traversal/percent-encoding ambiguity and alternate crate coordinates;
• keep exact credential-free HTTPS origin confinement;
• add negative tests for same-origin/wrong-crate requests;
• ensure the credential broker independently scopes or rechecks the crate resource rather than relying only on host isolation;
• update the interface contract if multiple Cargo URL forms need an explicit resource-path template or request kind.

Until this is specified, private Cargo fallback should remain fail-closed for URL shapes that cannot be proven resource-bound.