Consider canonical serialization format for Action hashing stability

[coderabbitai]

Problem

The current Action hashing implementation in src/ir.rs uses manual byte sequence construction for deduplication. This approach may be fragile and could break if the Action struct evolves, potentially causing hash instability across code changes.

Proposed Solution

Adopt a canonical serialisation format (e.g., serde JSON canonical) for hashing Action objects to ensure:

• Hash stability across code changes
• More maintainable serialisation logic
• Reduced risk of hash collisions from manual byte construction

Context

This suggestion arose from code review of the IR generation implementation.

Related:

• PR: Implement IR generation and validation #33
• Comment: Implement IR generation and validation #33 (comment)
• Issue: Consider canonical serialization format for Action hashing stability #35

Requested by: @leynos