Accept keyword datums in quotes

[pmatos]

Summary

• Add a Keyword ValueNode (analogous to Symbol) so quoted keyword literals like '#:foo parse and evaluate. The lexer already emitted Tok::KEYWORD for #:foo, but parseValue had no branch for it, so (linklet () () '#:foo) failed to parse.
• parseKeyword consumes the KEYWORD token; the node stores the bare keyword and prints in Racket's read form (#:foo). Keywords are not self-quoting, so the enclosing QuotedExpr adds the leading quote — '#:foo prints as '#:foo, and '(#:foo 1) as '(#:foo 1).
• Wire the new node through the ASTVisitor interface (Interpreter, AnalysisFreeVars).

Closes #72

Test plan

• ctest --preset debug — 14/14 unit tests pass, incl. new "Lexing Keyword tokens"
• nora-lit test/integration — 50/50 pass, incl. new quote19.rkt ('#:foo) and quote20.rkt ('(#:foo 1))
• cmake --build --preset debug — warning-clean (warnings-as-errors)
• clang-format — clean on all edited files

[codecov]

Codecov Report

❌ Patch coverage is 83.87097% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.58%. Comparing base (1ca9b29) to head (c3ced0c).

Files with missing lines	Patch %	Lines
src/Interpreter.cpp	0.00%	3 Missing ⚠️
src/AST.cpp	50.00%	1 Missing ⚠️
src/AnalysisFreeVars.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   73.19%   73.58%   +0.38%     
==========================================
  Files          24       24              
  Lines        2108     2139      +31     
  Branches      290      292       +2     
==========================================
+ Hits         1543     1574      +31     
  Misses        565      565              

Flag	Coverage Δ
integration	73.58% <83.87%> (+0.38%)	⬆️

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[claude]

Code review

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