feat(demo/stellar): Soroban RPC retry + exponential backoff (#59)

[KaruG1999]

Description

This PR introduces a centralized resilience primitive (withRetry) to wrap network interactions with Horizon and Soroban RPC nodes, mitigating transient failures (408, 429, 502, 503, 504) and generic network drops (TypeError).

The core implementation leverages an exponential backoff algorithm coupled with a full random jitter formula to prevent stampeding or herd effects on infrastructure endpoints under load.

Technical Changes

Core Architecture (src/lib/stellar/retry.ts)

• Implements withRetry<T>(fn, opts) using the exponential backoff calculation: $\min(\text{baseDelayMs} \times 2^{\text{attempt}-1} + \text{rand}(0, \text{baseDelayMs}), 30000)$.
• Targets specific transient HTTP status codes via RETRYABLE_STATUS set and native TypeError boundaries. All other 4xx errors abort immediately.
• Integrates native AbortSignal listeners to clear active timeouts and short-circuit thread execution instantly during component unmounts.

Component Integration & Idempotency Guardrails

• Aisolation of Write Operations: Explicitly excludes all POST /transactions submission endpoints from the retry loop across StellarSend.tsx and StellarReceive.tsx to prevent non-idempotent submission hazards (double-spending/collisions).
• Stealth Account Check Isolation: Enclosed the fetchWithRetry check inside an isolated try/catch/finally block in StellarSend.tsx. Persistent node degradation falls back to stealthExists = false instead of breaking the payment thread, preserving the original createAccount routing logic.
• UI State Cleanup: Added a finally block to the best-effort Soroban propagation context to prevent the "Retrying (3/3)…" string from freezing on successful payment rendering screens.
• Simulation Lifecycle: Wired AbortSignal directly into the debounce simulation effect within StellarSend.tsx to kill up to 6 orphan RPC queries on rapid input mutation.

Verification Results

Unit Testing (src/lib/stellar/retry.test.ts)

• 22/22 passing specs verified using Vitest and MSW v2.
• Verified execution paths for: early aborts, transient status code retries, validation failure rejections, and backoff boundary limits.

Compilation

• Executed local TypeScript checks via tsc --noEmit. Zero errors introduced in modified file paths.

closes #59

Any feedback is welcome, thankss!!

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[truthixify]

Clean merge. RPC retry + exponential backoff (retry.ts + 224-line tests + integration in StellarReceive/Send/Simulation) is solid. Vitest config + jitter handling all there. Thanks @KaruG1999.