Add lift and unlift commands for NSV line-level escaping

[namingbe]

Summary

This PR adds two new commands to the NSV CLI tool: lift and unlift. These commands apply NSV escaping/unescaping at the line level, allowing users to collapse or restore one structural dimension of NSV data.

Key Changes

• New lift command (alias l): Applies NSV escaping to each line, converting empty lines to backslash lines and escaping existing backslashes
• New unlift command (alias u): Applies NSV unescaping to each line, reversing the lift operation
• Both commands support reading from a file argument or stdin (when omitted or "-")
• Comprehensive test suite (tests/test_lift_unlift.sh) with 20+ test cases covering:
  • Empty input and single lines
  • Backslash escaping and unescaping
  • NSV structure preservation
  • Multiple empty lines
  • Trailing newline handling
  • Roundtrip verification (lift→unlift returns original)
  • Stdin input support
  • Command aliases
  • Existing NSV fixture files

Implementation Details

• Both functions follow the same pattern: read input, split on LF, apply the appropriate transformation to each line, and write with LF terminators
• Properly handles trailing newlines by treating them as terminators rather than creating empty lines
• Uses existing nsv::escape_bytes() and nsv::unescape_bytes() functions from the NSV module
• All output lines are guaranteed to have trailing newlines for consistency

https://claude.ai/code/session_01ETp6Za1KCCnFFsrvYQML4s