feat(parser): add line number support for $LINENO and error messages

[chaliy]

Summary

• Add comprehensive source location tracking throughout the parser and interpreter
• Implement working $LINENO variable that returns actual line number during execution
• Include line/column information in parse error messages

Changes

• New types: Position (line/column/offset) and Span (start/end positions) in parser/span.rs
• Lexer: Track line/column as tokenizing, emit SpannedToken with source locations
• AST nodes: All command-level nodes now include span field
• Parser: Thread span information from tokens into AST nodes
• Errors: New ParseAt error variant includes line/column in messages
• Interpreter: Track current execution line, return in $LINENO expansion
• Tests: New tests for multiline $LINENO, loops, and parse error locations

Test plan

• cargo fmt --check passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo test --lib --features http_client - 603 tests pass
• cargo test --test spec_tests - 13 tests pass
• New $LINENO tests verify multiline and loop line tracking
• Parse error tests verify location info in errors

https://claude.ai/code/session_01Gds1BKwWLE8SPEFN5txZfe

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