fix: resolve issues #147, #149, #150

Issue #147 - Final Doc-Comments and Rustdoc Generation:
- Add /// doc-comments to all 22 public items missing documentation
  in lib.rs, billing_dispute.rs, and lib_updated.rs
- Remove inline // comments that were blocking doc-comment detection
- Add scripts/check_doc_comments.sh CI script that fails the pipeline
  if any pub fn or pub struct lacks a doc-comment
- Script now passes: 'OK: All public items have doc-comments.'

Issue #149 - Write DISASTER_RECOVERY.md Runbook:
- Comprehensive emergency runbook covering 5 failure scenarios:
  Exploit/Fund Drain, Oracle Manipulation, Governance Attack,
  WASM Upgrade Gone Wrong, Stellar Network Outage
- Exact CLI commands for emergency_pause via multi-sig
- Multi-sig coordination protocol (3-of-5 Security Council)
- V1 → V2 state migration procedure with solvency invariant
- Post-incident checklist

Issue #150 - Final End-to-End Test:
- Add test_e2e_lifecycle.rs covering the complete subscriber lifecycle:
  Trial → Active → Pull → Upgrade → Dunning → Cancel
- Verifies state transitions at each stage
- Dumps final state ledger to prove total solvency invariant:
  subscriber + merchant + contract == initial_mint

Closes #147, #149, #150

Author: DanielCharis1