Add SDK request trace correlation IDs

[Kingsman-99]

Label: complexity: high
Points: 200

Description

For debugging multi-step flows (simulate → sign → submit → poll), correlating log lines/telemetry entries across the steps of a single logical operation needs a shared trace ID, currently absent.

Technical Context

New src/tracing.ts. Generates a trace ID per top-level public client call (pay, cloneInvoice, etc.), threading it through internal calls and into src/telemetryCollector.ts records and Issue 12's logger context.

Acceptance Criteria

• Each top-level StellarSplitClient public method generates a fresh trace ID for its execution
• Trace ID is included in TelemetryCollector.recordMethod calls and any logger context within that call's scope
• Nested internal calls (e.g. _submitTx within pay) inherit the parent's trace ID rather than generating their own
• Tests: trace ID consistency across a multi-step simulated call; independent calls get independent IDs

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