Centralize CSV/style option parsing

[eitoball]

Summary

Centralize parsing of to_comma style vs. CSV options so Generator and the Rails render csv: renderer share one code path.

Problem

Callers can pass either a style symbol or a hash of options:

users.to_comma(:short)
users.to_comma(style: :short, col_sep: ';', write_headers: false)
users.to_comma(filename: 'export.csv')

Today this is handled in two places with slightly different logic:

1. Comma::Generator#initialize (lib/comma/generator.rb) — treats style as either a Symbol or Hash, pulls out :style, :filename, passes remainder to Ruby's CSV library

2. Rails renderer (lib/comma.rb) — slices options from render kwargs, includes a string-to-boolean hack for write_headers:

   # XXX: Convert string to boolean
   h[k] = case k
          when :write_headers
            (v != 'false') if v.is_a?(String)
          else
            v
          end

This duplication is easy to drift (e.g. new Comma-specific option added in one place only).

Proposed approach

Add Comma::Options (or similar) with a single entry point:

Comma::Options.parse(input)
# => { style: :default, filename: nil, csv: { col_sep: ',', write_headers: true, ... } }

Use it from:

• Comma::Generator#initialize
• ActionController :csv renderer registration in lib/comma.rb

Document supported keys in one module (align with Comma::DEFAULT_OPTIONS in lib/comma.rb).

Files likely involved

• lib/comma/generator.rb
• lib/comma.rb
• New: lib/comma/options.rb (name TBD)
• spec/comma/comma_spec.rb (options / filename / write_headers examples)
• spec/controllers/users_controller_spec.rb (renderer option passthrough)

Acceptance criteria

• Comma::Options.parse (or equivalent) handles symbol, hash, and default cases
• String "false" for write_headers still works in controller specs
• filename, col_sep, force_quotes, style combinations unchanged (existing specs pass)
• Generator and renderer both delegate to the shared parser
• No public API change

Labels (suggested)

refactor

Depends on

None strictly; pairs well with #4 (Rails extraction).