Add CLI flags for condition binning strategy (linear vs quantile)

[aplgr]

Goal

Make condition/value binning tunable via CLI flags so users can choose how values are grouped into color buckets.

Proposed flags

• -binning linear|quantile (default linear)
• -bins N (default e.g. 10)

Expected behavior

• linear: buckets are evenly spaced between min/max (excluding sentinel values like “no data” / “dead” if applicable).
• quantile: buckets contain ~equal number of points each.

Acceptance criteria

• Both strategies produce stable, deterministic buckets for the same input CSV.
• Help text explains what each strategy means.
• README is updated with examples:
  • grovegrid ... -binning linear -bins 12
  • grovegrid ... -binning quantile -bins 8

Notes

Keep it simple; more advanced strategies can follow as separate issues (custom breakpoints, log scale, etc.).