feat(compliance): add tiered allowlist with allow_address_with_tier

[omonxooo-commits]

Summary

Adds on-chain compliance tier support to the compliance contract.

Changes

allowlist.rs

• New DataKey::Tier(Address) variant for per-address tier storage.

lib.rs

• allow_address_with_tier(env, admin, address, tier: u8) — allows an address and records its KYC/AML tier (0 = basic, higher = enhanced/institutional). Removes any existing expiry so the allow is permanent. Enforces admin auth and paused guard.
• get_address_tier(env, address) -> u8 — returns the stored tier, defaulting to 0 if unset.

compliance_test.rs

• 5 new tests covering: tier is set and address is allowed, default tier is 0, tier overwrites, non-admin rejected, paused contract rejected.

Design notes

• Tier storage is independent of the Allowed flag — is_allowed logic is unchanged.
• The treasury contract can call get_address_tier to enforce per-tier settlement limits without off-chain checks.

Closes #71

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