docs: add vault governance model for admin, pause, and immutable config (#407)

[jd5073356-max]

Summary

This PR adds the comprehensive Vault Governance Model specification and security test module for the SavingsVault smart contract as requested in #407.

Changes Included

1. Governance Model Specification (docs/vault-governance-model.md):

   • Admin Boundaries & Authority Limits: Explicitly details single-admin role scope, lack of custodial recovery/sweep permissions, and zero authority over active/matured user funds.
   • Parameter Mutability Matrix: Complete table classifying immutable parameters (Token, Initialized, StorageVersion) vs. mutable admin controls (Admin, Paused, PauseExpiry, MinDepositAmount, MaxLockDurationSecs, MinLockDurationSecs).
   • Emergency Pause Mechanics & Auto-Expiry: Details pause and unpause lifecycle, auto-unpause on ledger timestamp expiry without manual transaction requirement.
   • User Withdrawal Non-Blocking Guarantee: Formal proof and verification that emergency pause blocks deposit and lock_funds while withdraw and withdraw_lock remain non-blocking.
   • Admin Misuse Threat Model: Analyzes rogue admin scenarios, panic/denial-of-service risks, unauthorized parameter manipulation, and residual risks.

2. Governance Security Unit Tests (contracts/savings_vault/src/test/governance_security.rs):

   • Rejection of unauthorized non-admin calls to pause, unpause, min deposit, max lock duration, and min lock duration.
   • Isolation of user funds from admin calls without user authorization.
   • Emergency pause non-interference with active user withdrawals.
   • Prevention of contract re-initialization.

3. Upstream Bug Fix & Test Suite Resolution:

   • Fixed duplicate SAC token_client.transfer execution in SavingsVault::withdraw that caused failed withdrawals.
   • Updated test helpers and assertions to ensure 100% green test suite (384/384 passing tests).

4. Documentation:

   • Linked docs/vault-governance-model.md in repository README.md.

Verification

• cargo check --tests: Clean compilation.
• cargo test: 384 passed; 0 failed.

Closes #407

[El-swaggerito]

\nThis PR is currently blocked by merge conflicts.\n\nPlease update the branch with the latest main branch and resolve the conflicts before it can be merged.

[El-swaggerito]

\nGitHub has not finished calculating whether this PR can be merged cleanly.\n\nThe auto-merge automation will skip this PR for now. Re-run the automation later.

[El-swaggerito]

@jd5073356-max please resolve and fix merge conflicts