TxTracking should not modify the client's options object

The TxTracking system should not modify the client's `options` object that is supplied to every Arc.js function that generates a transaction. Currently it inserts a property called `txEventContext`.

This change may require adding an optional argument to every Arc.js function that generates a transaction.

The only known alternative is to document that Arc.js adds this property, and perhaps rename the property to be more clear about where it came from.

Start by locating the TxTracking implementation and the Arc.js functions that generate transactions. Trace where txEventContext is inserted into the client's options object and review how those functions receive options. Done means the issue's chosen approach is consistently applied without unintended mutation, with coverage or documentation matching the decision.

Written by the indexing model from the issue text.