Port validation, walk, and parser to arena IR

[Kroisse]

Summary

Port IR validation, walker, and text parser to arena IR, enabling use-chain consistency checks and printer↔parser round-trip.

Scope

• validation.rs: Port validation rules, add use-chain consistency checks
  • Verify every use points to a valid op
  • Verify every value's uses list is accurate
• walk.rs: Port IR walker/visitor to arena refs
• parser.rs: Port text IR parser to produce IrContext (~2,262 lines)
  • Parse text format → IrContext (instead of Salsa IR)

Key Files

• crates/trunk-ir/src/validation.rs
• crates/trunk-ir/src/walk.rs
• crates/trunk-ir/src/parser.rs (~2,262 lines)

Acceptance Criteria

• Validation checks use-chain consistency
• Walker traverses arena IR correctly
• Parser produces IrContext from text format
• Printer↔parser round-trip tests pass
• Existing validation/parser tests ported

[Kroisse]

All acceptance criteria met:

• ✅ Validation checks use-chain consistency (bidirectional)
• ✅ Walker traverses arena IR correctly
• ✅ Parser produces IrContext from text format
• ✅ Printer↔parser round-trip tests pass
• ✅ Existing validation/parser tests ported

Implemented across commits in #460 and the follow-up branch feature/arena-signature-conversion.