feat: add execution and replay modes to TransactionScheduler

[bmuddha]

Problem:
The scheduler currently handles all transactions uniformly. For replication, we need a distinct replay mode that enforces strict ordering based on transaction index from the primary, ignoring any other transactions that might come in.

Proposal:
Add a mode switch to TransactionScheduler (execution vs replay). In replay mode:

• Enforce strict ordering based on incoming transaction index field
• Ignore any transaction not marked for replay (log warning)
• Allow runtime mode switching without restart
• Execution mode remains unchanged (current behavior)

Acceptance criteria:

• Scheduler mode enum (Execution/Replay)
• Runtime mode switch method
• Replay mode enforces index-based ordering
• Non-replay transactions rejected with warning in replay mode
• Execution mode behavior unchanged

Reference: MIMD-0019 Client replay

Parent epic: #935