fix(engine): restore legacy mana spend triggers - #7387

The PR adds backward-compatible deserialization for legacy ManaSpellGrant save data in crates/engine/src/types/mana.rs. Changes are limited to: (1) a new LegacyManaSpellGrantRepr enum with a TryFrom conversion to the current ManaSpellGrant type, (2) checked i32::try_from(value) for mana-value thresholds with a test for out-of-range values, and (3) unit tests covering the migration paths. No CI/CD files, lifecycle hooks, dependency changes, network calls, or eval/exec patterns are present.