Deduplicate column argument parsing between extractors

[eitoball]

Summary

Extract shared column-argument parsing logic from DataExtractor and HeaderExtractor into a single code path.

Problem

Comma::DataExtractor#method_missing and Comma::HeaderExtractor#method_missing implement nearly identical branching over argument shapes:

Argument shape	Meaning
(none)	Bare method call on the instance
String	Custom header label; data still from the method
Symbol	Traverse association, call method on associated object
Hash	Association traversal with explicit header keys

Only the per-column handler differs (extract value vs. humanize header). Duplication means:

• Bug fixes must be applied twice
• New column types require two parallel changes
• Higher cognitive load for contributors

Relevant files:

• lib/comma/data_extractor.rb
• lib/comma/header_extractor.rb
• lib/comma/extractor.rb (base class)

Proposed approach

Introduce shared parsing on Comma::Extractor, e.g.:

def each_column_definition(sym, args)
  if args.blank?
    yield ColumnDefinition.new(source: :instance, method: sym)
    return
  end

  args.each do |arg|
    yield parse_column_arg(sym, arg)
  end
end

Each subclass supplies callbacks:

• DataExtractor — resolve value (instance, association, block transform)
• HeaderExtractor — resolve label (humanizer, custom string, association class)

Keep __static_column__ as separate entry points; data and header semantics differ intentionally.

Acceptance criteria

• All specs in spec/comma/data_extractor_spec.rb pass unchanged (behavior preserved)
• All specs in spec/comma/header_extractor_spec.rb pass unchanged
• All specs in spec/comma/comma_spec.rb pass unchanged
• Argument parsing logic exists in one place (DRY)
• No change to the public DSL

Out of scope

• Replacing method_missing with an explicit builder API (would be a major version change)

Labels (suggested)

refactor, good first issue (after #1 or in parallel if comfortable with extractors)

Depends on

Optional: #1 (STI fix) — unrelated but reduces noise when running AR specs during this work.