Use lossless value literal format in chain files

[colinrozzi]

Summary

• Bump packr to git branch with new value literal parser
• Chain format values now use lossless syntax (type-suffixed numbers, escaped strings)
• Add ParsedEvent::parse_input(), parse_output(), parse_state(), parse_params() methods that parse value literals back into Value

This builds on PR #31's chain format, making it fully lossless — chain files can now be parsed back into Value objects for replay.

Depends on: colinrozzi/pack#13

Test plan

• 30 lib tests pass
• Chain tests pass
• Compiles cleanly

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