Issue a node's bus accounts and grant when its assignment comes to stand (issue 1945, ADR 0543)

ADR 0539 clause 7 issues a module's credential and grant only when its build is done.
A module built long before and assigned to one more node never has a build done again:
on 2026-10-25 at 17:19 mesh-build-node and mesh-build-go were assigned to ace and composed
for it, the bus's user list never named ace's accounts, and both runtimes restarted every
5 s (one with no credential, one refused by the bus). The assign act composes the assigned
node alone, never the node holding the bus, and writes no grant.

So a write of the assignments bucket is the event: when a key comes to stand (new, or
ended or pending before), the lease holder issues the module's accounts on that node that
are not minted (never re-issued), composes the node holding the bus and writes the grants,
through the composer the done keys' take-ins use. A rewrite of a standing assignment does
nothing; the start's look settles every standing one, one after another. A step of
research 072's grants (memberships and credentials heard from placement).

internal/inventory/chain_assignments.go

@@ -361,3 +361,13 @@ func (i *Inventory) AssignmentValue(ctx context.Context, node, module string) (c
 	k, found, err := c.Assignments.Get(ctx, assignmentKey(node, module))
 	return k.Value, found, err
 }
+
+// WatchAssignments is the assignments bucket's raw entries, as the chain's watch gives them: every current value, a Caught
+// mark, then each write (novox/hq ADR 0543: an assignment that comes to stand issues its node's bus accounts).
+func (i *Inventory) WatchAssignments(ctx context.Context) (<-chan chain.Entry, error) {
+	c, err := i.theChain(ctx)
+	if err != nil {
+		return nil, err
+	}
+	return c.Assignments.Watch(ctx)
+}