Interpret #%variable-reference core form

[pmatos]

Summary

• Add a VariableReference AST value node and parse the three fully-expanded
  forms of the core syntax: (#%variable-reference),
  (#%variable-reference id), and (#%variable-reference (#%top . id)).
• The interpreter evaluates it to itself — an opaque, self-evaluating value that
  prints as #<variable-reference>, matching Racket v9.2.
• Wire the new node through the ASTVisitor interface (Interpreter,
  AnalysisFreeVars) and the parser (parseVariableReference, tried in
  parseExpr before parseApplication).

This form appears throughout expander/expander.rktl (always as
(variable-reference-from-unsafe? (#%variable-reference))); previously the
parser had the #%variable-reference token but no production for it, so these
expressions failed to parse.

Fixes #13

Test plan

• ctest --preset debug — Catch2 + lit/FileCheck, 14/14 passing
• New unit test Parsing variable references covers all three forms and the
  non-match (plain application) case
• New integration tests variable-reference{,1,2}.rkt — each prints
  #<variable-reference>
• Warning-clean build (warnings-as-errors) and clang-format clean

[codecov]

Codecov Report

❌ Patch coverage is 76.82927% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.61%. Comparing base (6922772) to head (e0820ac).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Parse.cpp	75.00%	12 Missing ⚠️
src/AST.cpp	14.28%	6 Missing ⚠️
src/AnalysisFreeVars.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #77      +/-   ##
==========================================
+ Coverage   74.44%   75.61%   +1.16%     
==========================================
  Files          24       24              
  Lines        2270     2391     +121     
  Branches      311      332      +21     
==========================================
+ Hits         1690     1808     +118     
- Misses        580      583       +3     

Flag	Coverage Δ
integration	75.61% <76.82%> (+1.16%)	⬆️

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[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

[pmatos]

Regarding the codecov coverage report (patch coverage 77.10%, 19 lines missing):

Deferred (diminishing returns) — Of the 19 uncovered lines, the src/AST.cpp (6) lines are the debug-only dump() method and the src/AnalysisFreeVars.cpp (1) line is a no-op visitor not wired into any runtime path — both are intentionally left uncovered here, matching existing repo convention. The src/Parse.cpp (12) lines are the parseVariableReference error-recovery branches; those are genuinely testable, but the three success paths and the non-match case are already covered by unit + integration tests, the codecov/patch check is green, and overall project coverage rose (+0.14%), so covering the malformed-input branches is a low-value follow-up rather than PR-blocking work.

Tracked as #86 (#86).

This assessment was made by two independent AI reviewers (Claude Opus 4.8 and Codex GPT-5.5). If you disagree, please reply and we'll re-evaluate.

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: e0820ac3d5

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[chatgpt-codex-connector]

Validate referenced ids before returning a variable reference

For the operand forms, the Racket reference says #%variable-reference represents the location of an id that must be bound as a variable (https://docs.racket-lang.org/reference/Locations____variable-reference.html), but this visitor ignores VR.getId() and always succeeds. With input such as (#%variable-reference missing), interpretation now silently returns #<variable-reference> instead of reporting an unbound variable/reference error, so invalid expanded code is accepted.

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