Refactor main.go to reduce cyclomatic complexity (gocyclo warnings)

[aplgr]

Goal

Reduce cyclomatic complexity in:

• cmd/grovegrid/main.go function main() (reported complexity 36)
• cmd/grovegrid/main.go function parseCSV() (reported complexity 19)

Target: get both below the Go Report Card / gocyclo warning threshold (>15), or at least significantly closer while keeping behavior identical.

Why

High complexity makes changes risky and discourages contributors. Smaller functions with clear responsibilities are easier to test and evolve.

Scope

Behavior-preserving refactor (no feature changes):

• Extract CLI parsing / validation into a small function (e.g. parseFlags()).
• Extract filesystem concerns (discover CSV files, sort slices, create output dir).
• Split parseCSV() into smaller steps:
  • open/read
  • delimiter detection
  • header normalization
  • row mapping + validation
  • numeric parsing + condition mapping
• Consider moving parsing helpers into a dedicated file/package (even within cmd/grovegrid/ is fine).

Acceptance criteria

• Output for the same input data is identical (HTML + optional JSON), excluding irrelevant formatting differences
• go test ./... passes
• gocyclo warnings are reduced (ideally no warnings > 15 for these functions)
• No new dependencies introduced

Notes / hints

If tests are missing, it’s okay to add a small regression test first (even one fixture) before refactoring aggressively.