feat: [DSM-142] Use CanisterStates in ReplicatedState

[alin-at-dfinity]

Switches ReplicatedState::canister_states from a flat BTreeMap<CanisterId, Arc<CanisterState>> to CanisterStates, exposing the hot/cold partition to the rest of the system and migrating every caller.

ReplicatedState changes:

• canister_states field is now CanisterStates.
• Drop canisters_iter_mut(). Round-level callers move to hot_canisters_iter_mut() (skips the long tail of cold canisters); bulk callers move to canisters_for_each_mut / canisters_try_for_each_mut, which iterate every canister and re-establish the partition afterwards.
• Add hot_canisters_iter() for read-only hot-only iteration.
• Add repartition_canister_states(), called from StateManager::commit_and_certify after flush_checkpoint_ops_and_page_maps to drive canisters that went quiet during the round back into cold before checkpointing, so that replicas continuing through a checkpoint and replicas (re)starting from it agree on the partition.
• take_canister_states / put_canister_states now exchange the CanisterStates directly instead of going through a flat BTreeMap round-trip.
• Aggregator delegations: total_compute_allocation, memory_taken, total_canister_memory_usage, guaranteed_response_message_memory_taken, best_effort_message_memory_taken, callback_count now delegate to CanisterStates and run in O(|hot|).

state_manager:

• commit_and_certify calls state.repartition_canister_states() after flush_checkpoint_ops_and_page_maps and before tip handover.
• validate_eq_canister_states calls CanisterStates::validate_strict_split on the reference state to verify that the persisted partition matches what CanisterStates::new would produce on a fresh load.
• flush_checkpoint_ops_and_page_maps and switch_to_checkpoint switch from canisters_iter_mut to canisters_for_each_mut / canisters_try_for_each_mut.
• Bench: bench_traversal likewise.

execution_environment:

• scheduler.rs: scheduler hot-only iteration where appropriate (add_heartbeat_and_global_timer_tasks, purge_expired_ingress_messages, the ongoing_long_install_code check); migrate charge_canisters_for_resource_allocation_and_usage and the log-memory-store migration loop to canisters_for_each_mut.
• round_schedule.rs: partition_canisters_to_cores now takes / returns a CanisterStates; idle canisters are dropped before the main hot-canister iteration.
• query_handler.rs, execution_environment.rs: callers updated.
• canister_manager/tests.rs, scheduler tests (scheduling.rs, metrics.rs, dts.rs, ecdsa.rs, round_schedule/tests.rs, test_utilities.rs, tests.rs) updated.
• benches/scheduler.rs: updated.

canonical_state:

• lazy_tree_conversion.rs: new CanisterStatesFork<'_> that presents a CanisterStates as a LazyFork over the merged hot+cold pools in CanisterId order.

messaging:

• stream_builder/tests.rs, state_machine/tests.rs, tests/common/mod.rs: caller updates.

replicated_state queues and system_state:

• CanisterQueues / SystemState local_canisters parameter type flips from &BTreeMap<CanisterId, Arc<CanisterState>> to &CanisterStates (no behavioral change; queues only need contains_key).

metrics.rs:

• check_dts walks hot_canisters_iter() (only hot canisters can have non-empty task queues).
• check_subnet_memory_usage switches to CanisterStates::memory_taken() for O(|hot|) aggregation.

test_utilities and state_tool:

• test_utilities/execution_environment, test_utilities/state, and state_tool/src/commands/canister_metrics.rs updated to use the new iteration APIs.