[ocakciomursel-pixel]

Closes #1037

Summary

Adds complete Soroban storage TTL management to the meridian-musdc-token contract so neither the token instance nor individual holder share balances silently archive while backing active vault positions.

Changes

• Instance storage TTL (30d bump / 29d threshold, matching adapter-common):
  • Extends instance TTL during initialization (init_state, shared by __constructor and initialize) and on every public entry point (mint, total_supply, admin, extend_ttl, and the full SEP-41 TokenInterface surface: allowance, approve, balance, transfer, transfer_from, burn, burn_from, decimals, name, symbol).
• Persistent balance storage TTL (120d bump / 113d threshold, matching MeridianVault POSITION_BUMP / POSITION_THRESHOLD):
  • Extends DataKey::Balance(addr) on every write (write_balance) and on existing-key reads (read_balance via if let Some(balance)), ensuring share balances share the exact 120-day lifetime window of the Entry/Principal records they back.
  • Guards missing keys before calling persistent().extend_ttl so balance() continues returning 0 for never-funded addresses and first-time mints/transfers never trap in soroban-env-host.
• Temporary allowance storage TTL:
  • Extends DataKey::Allowance(from, spender) for expiration_ledger - env.ledger().sequence() ledgers when recording positive allowances so multi-day approvals live through expiration_ledger instead of archiving after the network minimum temporary TTL.
• Permissionless keeper entry point (extend_ttl(env, holder: Option<Address>)):
  • Mirrors MeridianVault::extend_position_ttl so off-chain keepers or dormant holders can refresh instance storage and an optional Balance(holder) entry without authorization, safely skipping persistent extension when holder has no balance record.
• Operations / migration docs (apps/docs/operations/mainnet-deployment.md):
  • Documents the mUSDC TTL constants, why mUSDC and MeridianVault redeploy as an atomic pair (mUSDC exposes no upgrade entry point and MeridianVault fixes share_token at initialization), and how keepers pair vault.extend_position_ttl with musdc.extend_ttl. (packages/shared/deployed-contracts.json is intentionally untouched until an on-chain redeploy occurs.)

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p meridian-musdc-token --lib (32 unit tests passing, including 200-day multi-phase ledger archival survival, unbumped persistent entry archival trap verification, missing-key guards, permissionless extend_ttl, and temporary allowance TTL tests)
• cargo build --target wasm32v1-none --release
• prettier --check apps/docs/operations/mainnet-deployment.md

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