Include class variables in markdown output and index.csv

[skatkov]

Problem

Class variables (for example @@rubber_ducks) are currently missing from generated markdown and index.csv in rdoc-markdown, while previous YARD-based output indexed them.

Desired outcome:

• class variables are discoverable in generated docs,
• class variables are represented in index.csv.

Constraints and Investigation Notes

• RDoc parser does not expose class variables in the same way as constants.
• In Ruby parser logic, constant capture is gated by /^\\w+$/, which excludes @@... names.
• This likely requires either:
  • using hidden/available parser metadata if present, or
  • a supplemental extraction pass (Prism/Ripper-based) mapped to class/module context.

Proposed Tasks

• Add failing tests first (fixture expects class variable entry + anchor).
• Investigate available data in RDoc store for class variables across supported versions.
• Implement extraction strategy if class variables are not available from store.
• Emit class-variable section/anchors in markdown docs.
• Emit corresponding index.csv rows (proposed type=Constant for compatibility with existing consumers).
• Keep anchor format deterministic and encoded consistently.
• Add link/anchor validation coverage for new class-variable rows.

Example Expected Entry

Duck.@@rubber_ducks,Constant,Duck.md#classvariable--40-40rubber_ducks

Acceptance Criteria

• Class variables appear in markdown with valid anchors.
• Class variables appear in index.csv.
• Index links resolve to real anchors.
• Existing tests continue to pass.

[skatkov]

RDoc master comparison note: still not exposed by upstream RDoc store/output.

Tested with ruby/rdoc master at bc0baee787609f2205e8f1103f3e1d36b923184a. A minimal probe with @@rubber_ducks = 1 generated only the class page and class index row; no class-variable section, anchor, or index.csv row was available to this generator.

No exact upstream RDoc issue was found during triage. Unless upstream adds class-variable metadata, this likely needs local supplemental extraction.