feat: Credential delegation chain with depth-limited transitive trust and revocation propagation (#68)

[jotel-dev]

Description

Closes #68.

This PR implements multi-hop hierarchical credential delegation chains. Root issuers can delegate issuance authority to delegate addresses with depth constraints, trust fraction decay, circular delegation prevention, and cascading recursive revocation.

Changes Made

• Data Model: Added DelegationLink storing (parent, delegate, max_depth, trust_fraction, created_at, revoked).
• Storage Keys: Added DataKey::DelegationLink(Address, Address), DelegateChildren(Address), DelegateParent(Address).
• Trust Decay: Computes delegated trust level using fixed-point integer math across hops:
  $$\text{delegated_trust} = \text{root_trust} \times \frac{\text{fraction}_1}{10000} \times \frac{\text{fraction}_2}{10000} \times \dots$$
• Safety Features:
  • Circular delegation detection on create_delegation.
  • Basis points validation (trust_fraction <= 10000).
  • Cascading recursive revocation (revoke_delegation) propagating revocation to all descendant sub-delegates.
• Public Interface:
  • create_delegation(env, parent, delegate, max_depth, trust_fraction)
  • resolve_delegation_chain(env, delegate) -> Vec<Address>
  • compute_delegated_trust(env, delegate) -> u32
  • revoke_delegation(env, parent, delegate)
  • get_delegation_link(env, parent, delegate) -> DelegationLink
• Integration: Updated require_active_issuer and issuance methods to support delegated trust verification.
• Documentation: Documented delegation chain mechanics in docs/ARCHITECTURE.md.

Test Plan & Verification

• test_delegation_chain_trust_decay_and_revocation: Verifies multi-hop trust decay (80 -> 40 -> 32), credential issuance by delegates, and cascading revocation of sub-delegates.
• All unit tests passing via cargo test.