Add value literal parser for lossless round-tripping

[colinrozzi]

Summary

• Update Value::Display to emit type suffixes on numbers (42u32, -5s8, 3.14f32), proper string/char escaping, and remove list truncation — making the format lossless
• Add parse.rs with a hand-written recursive descent parser (FromStr for Value)
• Fully no_std compatible, supports all Value variants
• 15 unit tests including full round-trip verification

This enables lossless chain serialization: Display → FromStr → Display produces identical output, which is the foundation for chain replay from the human-readable format.

Format examples

42u32
"hello \"world\"\nbye"
actor-state{greeting: "Hello", count: 0u32}
color::rgb(255u8, 0u8, 128u8)
some("hello")
ok((actor-state{greeting: "Hi", count: 1u32}, "response"))

Test plan

• All 23 existing pack-abi tests pass
• 15 new parser tests pass
• Compiles in no_std mode (packr-guest)
• Round-trip test covers all Value variants

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