fix: make is_merchant_registered TTL-neutral (#563)

[Aj-Kayvee]

Summary

is_merchant_registered (public entry point) delegates to is_merchant_registered_internal, which extends the merchant entry's TTL on every call. This means a read-only oracle check mutates storage — an undesirable side effect that also exposes TTL manipulation through a public query.

Problem

The public is_merchant_registered (merchant.rs:87-92) guards on DataKey::Admin presence and then calls is_merchant_registered_internal, which unconditionally runs extend_ttl on the merchant persistent entry when the merchant exists. This causes:

1. TTL mutation on public read — a pure query side-effects storage.
2. Liveness oracle — callers can infer merchant liveness from TTL state.
3. Confusing side effects — a bool-returning public getter shouldn't extend storage lifetime.

Solution

Decouple the read path from the admin-warm path:

• New is_merchant_registered_read (storage.rs) — a pure persistent.has() check with no TTL bump. Used by the public entry point.
• is_merchant_registered_internal — unchanged. Continues to warm the merchant marker for internal callers (settlement, payments, admin migration) that intentionally keep entries alive.

Files changed

File	Change
settlement_contract/src/storage.rs	Add is_merchant_registered_read — TTL-neutral has() check
settlement_contract/src/merchant.rs	Switch public entry point from _internal → _read
settlement_contract/src/tests/admin_tests.rs	Add is_merchant_registered_is_ttl_neutral regression test

Regression test

is_merchant_registered_is_ttl_neutral registers a merchant, advances the ledger, then asserts that calling the public is_merchant_registered does not increase the merchant entry's TTL.

assert!(
    ttl_after <= ttl_before,
    "is_merchant_registered must be TTL-neutral"
);

Verification

cargo test --workspace
# 87 passed; 0 failed (86 existing + 1 new)

Closes #563

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