fix: resolve issues #751-754 — reconnect backoff, threshold checks, merge events, half-open probes

[Dev-sandy1]

Changes

• Reconnect dropped streams with exponential backoff in HorizonStreamManager #754 HorizonStreamManager: Reconnect dropped streams with exponential backoff (1s start, 30s cap). Emits stream:reconnecting with { attempt, delayMs } before each retry. Configurable maxReconnectAttempts (default: unlimited).
• Handle multi-key threshold calculation in AccountSignerWeightCalculator #753 AccountSignerWeightCalculator: Exported meetsThreshold(signers, threshold): Promise convenience method.
• Emit an event when AccountMergeDetector identifies a merged account #752 AccountMergeDetector: Emits a merge event with { source, destination, mergedAt } payload when a merge is detected, alongside the existing
  ecipient:mergeDetected event for backward compatibility.
• Add configurable half-open probe request count to CircuitBreaker #751 CircuitBreaker: Added configurable halfOpenProbeCount (default 1) allowing multiple concurrent probe requests in HALF_OPEN state. All probes succeeding closes the circuit; any failure re-opens it.

Closes #751
Closes #752
Closes #753
Closes #754

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