Add transaction result decoder, predicate builder, rate cache, and account flags inspector

[whisper011]

Summary

Adds four small, independent SDK features requested in issues #516–#519:

• Transaction result XDR decoder (src/txResultDecoder.ts) — parses the base64-encoded TransactionResult XDR returned by Horizon into a typed DecodedTransactionResult, including per-operation results and fee-bump inner/outer results. Wired into src/receipt.ts as PaymentReceipt.decodedResult.
• Claimable balance predicate builder (src/predicateBuilder.ts) — fluent PredicateBuilder for composing unconditional, absolute-window, relative-window, and AND/OR claim predicates without touching the raw Claimant/xdr.ClaimPredicate API. Wired into src/claimableBalanceFallback.ts.
• Exchange rate cache with TTL eviction (src/rateCache.ts) — RateCache serves rates from memory until TTL expiry, with a background refresh at 80% of TTL to avoid cold-fetch latency, plus invalidate/invalidateAll. Wired into src/currencyConverter.ts.
• Account flags state inspector (src/accountFlagsInspector.ts) — inspectFlags() decodes a Stellar account's AUTH_* flags into a typed AccountFlagSet with an isCompatibleWith(operation) check and a hasAnyRestrictiveFlag() convenience helper. Wired into src/preflightChecker.ts as checkRecipientFlags() so callers can preflight-check a recipient's flags before sending a payment or opening a trustline.

Each feature is scoped to its own commit.

Closes

Closes #516
Closes #517
Closes #518
Closes #519

Test plan

• npm run build
• npm test
• ESLint / TypeScript checks
• Manual sanity check: decode a known success and failure TransactionResult XDR fixture
• Manual sanity check: build an absolute-window and AND/OR predicate and pass to createClaimableBalance
• Manual sanity check: RateCache serves cached rate within TTL and refetches after expiry
• Manual sanity check: inspectFlags() against an AUTH_REQUIRED issuer account returns isCompatibleWith("payment") === false

Note: this repo currently has pre-existing TypeScript/build errors unrelated to this change (e.g. in client.ts, preflightChecker.ts's existing sponsor-reserve check, several __tests__ files) — these were present before this branch and are out of scope here.

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