`wraith-names` hierarchical subdomain support

[truthixify]

Tier: L (1-2 weeks) Type: feature

Context

Users will want subdomains under their .wraith names (e.g., owner of alice.wraith controlling payments.alice.wraith, team.alice.wraith). Today the name registry is flat. Add hierarchical delegation.

Scope

• Extend wraith-names to support <sub>.<parent>.wraith lookups
• Owner of parent name can register/update/release subdomains under it
• Up to 2 levels of nesting (parent + 1 sub) for simplicity
• Resolve payments.alice.wraith walks: lookup alice → check delegation → return sub-record
• Migration-safe: existing flat names still work

Acceptance criteria

• Contract changes with hierarchical resolution
• Tests for register/update/release on subdomain
• Tests for permission boundary (only parent owner can manage subs)
• SDK follow-up issue for hierarchical resolver
• Documentation update

Files to start with

• stellar/wraith-names/src/lib.rs