Cleanup: delete unreachable legacy State-based transition machinery (looser validation, no live caller)

[cryptskii]

Summary

Two state-transition paths coexist: the canonical per-device DeviceState::advance /
advance_relationship path, and an older State-based path (transition::apply_transition,
RelationshipManager::execute_relationship_transition, BilateralTransactionManager::execute_bilateral_transaction).
The older relationship.rs apply_transition performs no signature, balance, or token-consistency checks.

Severity: Low (cleanup). The older transition-execution machinery is not reachable from any live
SDK/JNI/bridge/Android/frontend code — it is wired only into a SDK example and integration tests. Production
uses only the free helpers compute_smt_key / initial_chain_tip_from_device_ids from that module. Per repo
zero-legacy policy this is dead protocol-bearing code that should be removed before someone wires it into a live
path (which would jump it to High).

Evidence (dsm_client/deterministic_state_machine/)

• Canonical path: dsm/src/core/state_machine/mod.rs:163 (prepare_advance_relationship), :216
  (commit_advance); dsm/src/types/device_state.rs:457 (DeviceState::advance, enforces token consistency
  with checked_add/checked_sub).
• Older path (looser): dsm/src/core/state_machine/relationship.rs:657 (apply_transition — clones state, swaps
  op/entropy, rehashes; no op-level validation); :538 (execute_relationship_transition);
  dsm/src/core/state_machine/transition.rs:774 (apply_transition), :881 (create_next_state);
  dsm/src/core/bilateral_transaction_manager.rs:874 (execute_bilateral_transaction).
• Reachability — older execution path is dead in prod: execute_bilateral_transaction callers are only the
  #[cfg(test)] module (bilateral_transaction_manager.rs:1814,1865), the SDK example
  dsm_sdk/examples/bilateral_transaction_example.rs:313,326, and integration tests
  dsm/tests/bilateral_transaction_integration_tests.rs. Live SDK uses prepare_advance_relationship +
  commit_advance (dsm_sdk/src/sdk/core_sdk.rs:856/933/1233,
  dsm_sdk/src/bluetooth/bilateral_ble_handler.rs:4876-4880).
• Note: create_next_state is now fail-closed on incoming unilateral transfers (transition.rs:907-924, the
  Issue core/state_machine: relationship keys still use Base32 text and unilateral receive can skip local signature verification #194 fix), so that specific looseness is already addressed; the genuine remaining looseness is the
  relationship.rs apply_transition helper.

Fix (cleanup, delete legacy)

1. Delete BilateralTransactionManager::execute_bilateral_transaction / exec_offline / exec_online /
   execute_transition_bytes, keeping only the live free functions (compute_smt_key,
   initial_chain_tip_from_device_ids, SMT-replace/anchor helpers).
2. Delete BilateralStateManager::execute_transition_bytes, RelationshipManager::execute_relationship_transition,
   and relationship.rs's local apply_transition (:657).
3. Delete transition::apply_transition/create_next_state once their tests are removed (verify still-live
   exports are untouched).
4. Rewrite the SDK example + integration tests against the canonical prepare_advance_relationship/commit_advance API.
5. If full deletion is too large for one change, gate the entire older module behind #[cfg(test)] / a non-default
   feature so no shippable build can reach it.

Affected files

• dsm_client/deterministic_state_machine/dsm/src/core/state_machine/{mod.rs,transition.rs,relationship.rs,bilateral.rs}
• dsm_client/deterministic_state_machine/dsm/src/core/bilateral_transaction_manager.rs
• dsm_client/deterministic_state_machine/dsm_sdk/examples/bilateral_transaction_example.rs
• dsm_client/deterministic_state_machine/dsm/tests/bilateral_transaction_integration_tests.rs