fix(StakeManager.spec): use filtered invariants for vacuous rules

This refactors the spec to no longer rely on the `simplification()`
but instead filter out the vacuous rules from the get go.

Using the `simplification()` previously was needed so that the prover
will ignore cases that revert by design. This made some invariants
vacuous.

Having vacuous rules or invariants is still considered a failure, so to
make get prover happy, we're using filtered invariants instead which
renders the `simplification` obsolete.

certora/specs/StakeManager.spec

@@ -39,6 +39,17 @@ function simplification(env e) {
   require e.msg.sender != 0;
 }
 
+definition requiresPreviousManager(method f) returns bool = (
+  f.selector == sig:migrationInitialize(uint256,uint256,uint256,uint256).selector ||
+  f.selector == sig:migrateFrom(address,bool,StakeManager.Account).selector ||
+  f.selector == sig:increaseMPFromMigration(uint256).selector
+  );
+
+definition requiresNextManager(method f) returns bool = (
+  f.selector == sig:migrateTo(bool).selector ||
+  f.selector == sig:transferNonPending().selector
+  );
+
 ghost mathint sumOfEpochRewards
 {
   init_state axiom sumOfEpochRewards == 0;
@@ -67,16 +78,14 @@ hook Sstore accounts[KEY address addr].currentMP uint256 newValue (uint256 oldVa
 
 invariant sumOfBalancesIsTotalSupplyBalance()
   sumOfBalances == to_mathint(totalSupplyBalance())
-      { preserved with (env e) {
+  filtered {
-          simplification(e);
+    m -> !requiresPreviousManager(m) && !requiresNextManager(m)
-        }
   }
 
 invariant sumOfMultipliersIsMultiplierSupply()
   sumOfMultipliers == to_mathint(totalSupplyMP())
-      { preserved with (env e) {
+  filtered {
-          simplification(e);
+    m -> !requiresPreviousManager(m) && !requiresNextManager(m)
-        }
   }
 
 invariant sumOfEpochRewardsIsPendingRewards()
@@ -88,9 +97,8 @@ invariant sumOfEpochRewardsIsPendingRewards()
 
 invariant highEpochsAreNull(uint256 epochNumber)
   epochNumber >= currentContract.currentEpoch => currentContract.epochs[epochNumber].epochReward == 0
-  { preserved with (env e) {
+  filtered {
-      simplification(e);
+    m -> !requiresPreviousManager(m) && !requiresNextManager(m)
-    }
   }
 
 rule reachability(method f)