feat(phase-2.3): indentation tracking with INDENT/DEDENT tokens

[Mjboothaus]

Phase 2.3: Indentation Tracking - LEXER COMPLETE!

This completes Phase 2 of the YAML Lite implementation. The lexer now has full indentation tracking.

Implementation Details

Core Logic:

• Track indentation level at start of each line using indent_stack
• Emit INDENT token when indentation increases
• Emit DEDENT token(s) when indentation decreases
• Handle edge cases: blank lines, comment-only lines, EOF

Features:

• ✅ Multiple nesting levels
• ✅ Multiple DEDENT tokens (jumping back multiple levels)
• ✅ Mixed structures (lists + mappings)
• ✅ Indentation validation (detect misaligned dedents)
• ✅ Blank line handling (don't affect indentation)
• ✅ Comment line handling (don't affect indentation)
• ✅ EOF handling (emit remaining DEDENTs)

Testing

10 new indentation tests:

• Simple indent/dedent
• Multiple nesting levels
• Multiple dedents at once
• Blank lines ignored
• Comment lines ignored
• List with indented items
• Same indent (no tokens)
• DEDENT at EOF
• Mixed indentation with lists

Updated 2 structural tests to account for INDENT/DEDENT tokens

Test Results

• 74 total tests (10 basic + 15 scalars + 7 structural + 10 indentation + 17 values + 15 fixtures)
• 100% passing

Phase 2 Complete! ✅

• ✅ Phase 2.1: Scalar tokenisation
• ✅ Phase 2.2: Structural tokens
• ✅ Phase 2.3: Indentation logic

Next Steps

Phase 3 will implement the parser to build YamlValue structures from the token stream.

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