Name who a read-write access grants its path's group to, so modules write shared data through the group the operator gave it (hq ADR 0312, issue 537)

Each read-write access is composed with a grant: the module's containers that
mount its path and the account its tools run as (the operator's, or the module's
own under ADR 0259 §8); that runtime restarts on the access, so it holds a group
the engine put its account in. A read access grants nothing. Sent only to an
engine whose statement says GrantContract (4), since an older one refuses the
field. Pins the node-engine at the branch that reads it.

internal/link/protocol.go

@@ -436,6 +436,12 @@ const ReadinessContract = 2
 // parses strictly and would refuse the whole declaration for it.
 const RootContract = 3
 
+// GrantContract is the statement of an engine that also reads an access's `grant` (novox/hq ADR 0312): it grants a
+// read-write access's path group to the containers and the account the grant names, takes it back, and states the
+// access unhealthy while the group cannot write. Only to such an engine is the field sent: an older one parses
+// strictly and would refuse the whole declaration for it.
+const GrantContract = 4
+
 // ReasonRoot starts the reason of an account verdict that found a way to root (ADR 0266); the node-engine's
 // own words (mesh-host internal/accounts ReasonRoot).
 const ReasonRoot = "can become root without a person"
@@ -487,6 +493,11 @@ const KindAccount = "account"
 // ReasonUsedAsFound.
 const KindDirectory = "directory"
 
+// KindAccess is a read-write access that grants its path's group (novox/hq ADR 0312): healthy while the group may
+// write the path, unhealthy while it cannot — the operator's to decide, since the mesh changes nothing on the path
+// — or while the path is owned by a group that is never granted. The node-engine's own (mesh-host internal/link).
+const KindAccess = "access"
+
 // ReasonUsedAsFound starts the reason of a directory used as found.
 const ReasonUsedAsFound = "used as found:"