fix(escrow): implement explicit legacy version and migration checks

[onyekachi66]

Closes #1216

Approach

This PR addresses the correctness and security gap where older storage lacking a version marker could corrupt live escrow state if its layout was incorrectly guessed. We introduce an explicit LEGACY_VERSION (5) and a typed error EscrowError::AmbiguousLegacyStorage (93). The migrate function now explicitly detects the absence of a version marker and verifies if the storage matches the known legacy layout (by checking for the existence of DataKey::FundingToken and DataKey::Treasury). If it doesn't match the known legacy layout, the ambiguous storage is rejected with AmbiguousLegacyStorage.

Test Evidence

We have added focused unit and integration coverage for the new behavior in escrow/src/tests/migration_errors.rs, covering every requested edge case:

• known legacy layout: test_known_legacy_layout_migration
• missing marker: test_ambiguous_legacy_storage_missing_marker
• unknown marker: test_unknown_marker
• migration repeated: test_migration_repeated
• partial migration: test_partial_migration

All existing tests pass with no regressions, and the new checks maintain atomic, bounded state changes with no dead code.

Security Notes

• Storage compatibility is strictly preserved.
• We rely on has(&DataKey::FundingToken) and has(&DataKey::Treasury) as the heuristic to identify a fully initialized V5 known legacy layout. If those keys are absent on a markerless instance, we reject it as ambiguous rather than guessing its layout, preventing corruption.
• The AmbiguousLegacyStorage error is structured, stable, and does not leak internal details.
• The existing auth and replay boundaries in migrate remain intact and precede the new version marker logic, ensuring safe concurrent invocation and replay resistance.

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