feat(governance): clear voting power snapshots on Mission 70 activation (#9731)

## Summary

- Add `VotingPowerSnapshots::clear()` method that empties both stable
BTreeMaps
- Call it during governance initialization when the Mission 70 flag is
enabled
- Add a log line when a voting power spike is detected for observability
- Add unit test for `clear()` verifying both maps are emptied

## Why

When Mission 70 activates, voting power distribution changes
significantly (convex dissolve delay bonus, reduced max dissolve delay).
Stale snapshots from before activation would cause false spike
detections, since the new voting power totals will differ substantially
from the old ones.

## Test plan

- [x] Unit test added for `VotingPowerSnapshots::clear()` verifying both
maps are emptied

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jasonz-dfinity

rs/nns/governance/src/governance.rs

@@ -8,6 +8,7 @@ use crate::{
             validate_merge_neurons_before_commit,
         },
         split_neuron::{SplitNeuronEffect, calculate_split_neuron_effect},
+        voting_power_snapshots::VotingPowerSnapshots,
     },
     heap_governance_data::{
         HeapGovernanceData, XdrConversionRate, initialize_governance, reassemble_governance_proto,
@@ -1355,6 +1356,8 @@ impl Governance {
             governance.heap_data.neuron_id_to_pre_clamp_dissolve_state = governance
                 .neuron_store
                 .clamp_dissolve_delay_for_all_neurons_or_panic(now);
+
+            VOTING_POWER_SNAPSHOTS.with_borrow_mut(VotingPowerSnapshots::clear);
         }
 
         governance

rs/nns/governance/src/governance/voting_power_snapshots.rs

@@ -5,7 +5,7 @@ use crate::{
     pb::v1::{Ballot, NeuronIdToVotingPowerMap, VotingPowerTotal},
 };
 
-use ic_cdk::eprintln;
+use ic_cdk::{eprintln, println};
 use ic_nervous_system_common::ONE_MONTH_SECONDS;
 use ic_stable_structures::{
     DefaultMemoryImpl, StableBTreeMap, Storable, memory_manager::VirtualMemory, storable::Bound,
@@ -202,6 +202,14 @@ impl VotingPowerSnapshots {
             voting_power_map,
             totals_with_minimum_total_potential_voting_power,
         ));
+        println!(
+            "{}Voting power spike detected at timestamp {}, total potential voting power: {}, \
+            minimum total potential voting power: {}",
+            LOG_PREFIX,
+            timestamp_with_minimum_total_potential_voting_power,
+            total_potential_voting_power,
+            totals_with_minimum_total_potential_voting_power.total_potential_voting_power
+        );
         Some((
             timestamp_with_minimum_total_potential_voting_power,
             previous_voting_power_snapshot,
@@ -214,6 +222,13 @@ impl VotingPowerSnapshots {
             .last_key_value()
             .map(|(timestamp, _)| timestamp)
     }
+
+    /// Clears the voting power snapshots.
+    // TODO(NNS1-4323): Remove this method after Mission 70 is fully deployed.
+    pub(crate) fn clear(&mut self) {
+        self.neuron_id_to_voting_power_maps.clear_new();
+        self.voting_power_totals.clear_new();
+    }
 }
 
 impl Storable for NeuronIdToVotingPowerMap {

rs/nns/governance/src/governance/voting_power_snapshots_tests.rs

@@ -113,3 +113,32 @@ fn test_record_voting_power_snapshot() {
         None,
     );
 }
+
+// TODO(NNS1-4323): Remove this test after Mission 70 is fully deployed.
+#[test]
+fn test_clear() {
+    let memory_manager = MemoryManager::init(DefaultMemoryImpl::default());
+    let mut snapshots = VotingPowerSnapshots::new(
+        memory_manager.get(MemoryId::new(0)),
+        memory_manager.get(MemoryId::new(1)),
+    );
+
+    // Record a few snapshots.
+    for i in 1..=3 {
+        snapshots.record_voting_power_snapshot(i, voting_power_snapshot(vec![90, 10], 100 + i));
+    }
+
+    // Verify that snapshots are present.
+    assert_eq!(snapshots.latest_snapshot_timestamp_seconds(), Some(3));
+
+    // Clear the snapshots.
+    snapshots.clear();
+
+    // Verify that everything is empty.
+    assert_eq!(snapshots.latest_snapshot_timestamp_seconds(), None);
+    assert_eq!(
+        snapshots.previous_ballots_if_voting_power_spike_detected(u64::MAX, 10),
+        None
+    );
+    assert!(!snapshots.is_latest_snapshot_a_spike(10));
+}