test(escrow): maturity gate boundary and zero-maturity behavior

[Edoscoba]

Summary

Adds explicit tests for the settle maturity boundary: one second before maturity traps without mutating state, exactly at maturity succeeds, and maturity == 0 allows immediate settlement.

Also surfaces has_maturity_lock in the read API and init event so operators and indexers can distinguish “no maturity lock” from a positive ledger-time lock, with ADR and ledger-time documentation updates.

Validation:

• cargo test
• cargo clippy -p liquifact_escrow --all-targets -- -D warnings
• cargo llvm-cov --features testutils --fail-under-lines 95 --summary-only -p liquifact_escrow

close #270

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