CST parser: lossless concrete syntax tree for .sx files
New sx_cst.ml: CstAtom, CstList, CstDict node types with leading/trailing
trivia (whitespace + comments). Two projections:
- cst_to_source/cst_file_to_source: exact source reconstruction
- cst_to_ast: strip trivia → Sx_types.value for evaluation
New parse_all_cst/parse_file_cst in sx_parser.ml: parallel CST parser
alongside existing AST parser. Reuses read_string, read_symbol, try_number.
Trivia collected via collect_trivia (replaces skip_whitespace_and_comments).
Round-trip invariant: cst_file_to_source(parse_all_cst(src)) = src
Verified on 13 synthetic tests + 7 real codebase files (101KB evaluator,
parser, primitives, render, tree-tools, engine, io).
CST→AST equivalence: cst_to_ast matches parse_all output.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
giles
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