refactor(quickadapter): disambiguate and centralize final exit stage constants

refactor(quickadapter): disambiguate and centralize final exit stage constants

Rename the _FINAL_EXIT_STAGE tuple to _FINAL_EXIT_STAGE_PARAMS and add the
class-level constant _FINAL_EXIT_STAGE_INDEX (max(partial_exit_stages) + 1),
referenced from the configuration logging, the get_trade_exit_stage clamp,
and the plot config. This removes the triplicated stage-index expression and
distinguishes the final stage parameters from its index. Qualify the plot
config partial_exit_stages and stage-index accesses with QuickAdapterV3 to
match the surrounding constant access.