Add progress bars (progressr) for af()/vis()/bglmnet() bootstrap loops

[garthtarr]

Part of the parallelization improvement plan tracked in PARALLELIZATION.md (Priority 4, optional/advanced).

Split out of #16 so it can be scoped and closed independently of backend selection and core-count heuristics.

Background

af(), vis(), and bglmnet() all run furrr::future_map() bootstrap loops that can take anywhere from a few seconds to several minutes depending on B/n.c/nlambda, with no feedback to the user while running.

Tasks

• Add progressr support to af()'s future_map() call (the n.c-value bootstrap loop)
• Add progressr support to vis()'s future_map() call (the B-replicate loop)
• Add progressr support to bglmnet()'s future_map() call (the B-replicate loop)
• Decide on and document a consistent progress-bar UX across all three (e.g. a shared internal helper, opt-in/opt-out behavior, whether it's on by default in interactive sessions)
• Add progressr to Suggests (or Imports, depending on default-on/off decision) in DESCRIPTION
• Add a test confirming the functions still work correctly with a progressr handler registered (and with none registered)

See PARALLELIZATION.md section "Performance Considerations" and the Phase 4 checklist for context.