fix(ingestion): ensure replay idempotency and deterministic dedup

[Nwoyemartha]

Implements durable replay safety and idempotency for Horizon ingestion and reconciliation (issue #1261):

• SorobanClient: integrate SingleFlight into callRpc() with deterministic operation fingerprinting (SHA-256 of method + canonical JSON params) to coalesce concurrent and retried RPC calls into a single round-trip.
• horizonBondEvents: add conflicting event detection via resolveConflictingEvent() and ConflictingEventError — when the same event ID arrives with a materially different payload, it is rejected deterministically. Identical replays are safe no-ops that skip business logic but still advance the cursor.
• ReplayService: wrap each event replay in IdempotentConsumer to prevent duplicate business effects when replayLedgerRange() is re-run after a crash.
• Add canonicalJson() for deterministic, key-order-independent fingerprinting.
• Add throwIfCancelled() to SorobanClient for abort-signal fail-fast.

Correctness invariants:

• Same logical request can be retried safely (SingleFlight coalesces).
• Duplicate processing results in one committed business effect.
• Conflicting request-key reuse is rejected (ConflictingEventError).
• Failed/rejected operations leave no unauthorized partial state.
• Replay/reordering converges to authoritative state.