Add Display impl for Value and ValueType

[colinrozzi]

Summary

Add human-readable Display impls for Value and ValueType.

Value examples:

• 42, "hello", true — primitives
• (42, "hello") — tuples
• [1, 2, 3] — lists (truncated at 10 items)
• some(42) / none — options
• ok(42) / err("bad") — results
• actor-state{greeting: "Hello", count: 0} — records
• color::rgb(255, 0, 0) — variants

ValueType examples:

• string, u32, bool — primitives
• list<u8>, option<string> — generic types
• result<actor-state, string> — results
• tuple<s32, s32> — tuples

This enables better error messages in theater (currently dumps raw CGRF bytes).

Test plan

• All workspace tests pass
• Derive tests pass
• Clippy clean

🤖 Generated with Claude Code