Capabilities, principals & decisions

pc-core is the protocol-neutral hub every crate reasons about:

• Tenant — the top-level isolation boundary; a required parameter everywhere.
• Principal — an opaque identity. OIDC, mTLS, PAT, and SPIFFE are adapters onto the same type. Principals compose into a delegation chain (origin first, current delegate last), so policy can reason over chain depth and origin — nothing assumes a single human at the top.
• Capability — a protocol-neutral “thing that can be invoked”, pinned by the content hash of its full definition. Drift from the pinned hash quarantines it (rug-pull defence).
• Invocation — who is calling what, in which tenant, with what argument shape (a hash of keys/types, never values).
• Decision — the fail-closed outcome: anything not explicitly allowed is denied.

None of these name a wire protocol. A new protocol is a new pc-proto-* crate, not a change to the core.