fix(web): add readonly mode for rule behavior finalization

Relates to #5853.

Two things happened here:

1. Construction of Mocks made an assumption that the selection should
   always be deleted (outputTarget.ts:363). However, for NewContext and
   PostKeystroke processes, we don't want to change anything.

2. Even if nothing is changed, the transcription would emit what is
   in theory a no-op ruleTransform (insert="", deleteLeft=0,
   deleteRight=0). But apps would treat this as deleting the selection.

This fix goes a little broader than I would have preferred, but adds a
readonly mode to the transcription and mock model, so that we can
control explicitly when changes are applied to the text store.