Foundational identity and permission types for role-based access control — bit-set PermissionMask, immutable Identity value type, and PermissionProvider interface.
What's included:
- `Identity` value type with NewIdentity / WithTenant constructors and SetInContext / FromContext context helpers
- `Permission` (int64 bit position) and `PermissionMask` (int64 bit-set) with O(1) Has and non-mutating Grant
- `PermissionProvider` interface for DB-backed ResolveMask(ctx, uid, resource) resolution
Tested-via: todo-api POC integration
Reviewed-against: docs/adr/
