Offer a choice between showing T-states or M-cycles in opcode tables

Currently there are choices between themes (Light, Dark, or Classic) and widths (Standard or Octal) but durations are always shown in T-states, with a footnote that "each instruction takes a multiple of four T-states to complete". I would personally find M-cycles easier to have as the only option since there's no factor of 4 to think about, but a choice would also be useful.

Start by tracing how the existing theme and width choices are implemented for the opcode tables. Add an option that switches duration display between T-states and M-cycles, and update the explanatory footnote so it matches the selected mode; done means both choices render correctly.

Written by the indexing model from the issue text.