Measure a declaration against the bus's largest message, raise a condition near it, and send the vault's export compact (issue 1689)

A declaration is one bus message and the control node's passed the limit by 2 KB. The size is checked at every send
against the connection's own limit, with a condition from 90% and a message naming size and limit when over; the
export, 138 KB compact of 165 KB indented, is sent compact.

internal/link/declare.go

@@ -46,8 +46,41 @@ func Declare(ctx context.Context, bus Bus, signer Signer, node string,
 		return err
 	}
 
+	// **Measured before it is sent** (novox/hq issue 1689): the bare bus error said only "maximum payload
+	// exceeded", on 2026-10-21 for the control node's declaration, 2 KB over. The limit is the connection's own.
+	if limit := payloadLimitOf(bus); limit > 0 {
+		size := int64(len(body))
+		if DeclarationMeasured != nil {
+			DeclarationMeasured(ctx, node, size, limit)
+		}
+		if size > limit {
+			return fmt.Errorf("%s's declaration is %d bytes on the wire (%d bytes of declaration, signed and encoded), "+
+				"over the bus's largest message of %d bytes by %d: it cannot be sent (novox/hq issue 1689)",
+				node, size, len(declaration), limit, size-limit)
+		}
+	}
+
 	publish, cancel := context.WithTimeout(ctx, timeout)
 	defer cancel()
 
 	return bus.PublishDeclaration(publish, node, body)
 }
+
+// NearTheLimit is the share of the bus's largest message at which a declaration is said to be near it.
+const NearTheLimit = 0.9
+
+// DeclarationMeasured is told, at every send of a declaration, its size on the wire and the bus's largest message,
+// so the controller raises a condition when it nears the limit and clears it when it is back under (issue 1689).
+// Set by the controller; nil in a test or a tool.
+var DeclarationMeasured func(ctx context.Context, node string, size, limit int64)
+
+// payloadLimiter is a bus that knows its largest message.
+type payloadLimiter interface{ MaxPayload() int64 }
+
+// payloadLimitOf is the bus's largest message in bytes, or 0 when the bus does not say.
+func payloadLimitOf(bus Bus) int64 {
+	if l, ok := bus.(payloadLimiter); ok {
+		return l.MaxPayload()
+	}
+	return 0
+}