Preset-bank immutability

Currently in VST2 (and possibly AU), changing parameters and switching between presets modify presets in memory.
Doesn't affect user sessions.

Because of Studio One calling setProgram after setting a bank chunk, I wasn't able to make the preset bank immutable.
It is a life quality enhancement, since factory presets being modified is not a superb user experience.

Can not be done until all plugins are converted to chunks. Blocked by `legacyVST2Chunks`.

Start by investigating the VST2 and AU preset-handling paths, especially the legacyVST2Chunks blocker and the interaction between bank chunks and setProgram. The work is done when changing parameters or switching presets no longer modifies factory presets in memory without affecting user sessions, including the Studio One sequence described.

Written by the indexing model from the issue text.