feat: add cross-package transaction readiness engine with typed states and stages (Closes #21)

[deepak0x]

Description

Implements a cross-package Stellar transaction readiness engine (issue #21) — a single reusable surface that validates account state, asset, amount, memo, network mode, and submission safety before any transaction is built. Testnet-first; never submits real payments.

What changed

• packages/types/src/index.ts:
  • Added ReadinessState (ready | warnings | unsafe-network | blocked) — a typed aggregate outcome.
  • Added ReadinessStage ({ id, label, status, warnings }) — per-stage result.
  • Extended TransactionReadiness with state and stages (backward-compatible: ready/warnings/summary retained).
• packages/stellar-kit/src/intent.ts:
  • Refactored estimateTransactionReadinessSync to run 7 discrete, reportable stages (account-source, account-dest, asset, amount, memo, network, balance) instead of one flat warning list.
  • getReadinessState(warnings) derives the typed ReadinessState.
  • mapReadinessToErrorCode(warnings) returns the first error code for logging/branching.
  • estimateTransactionReadiness (async) unchanged signature; now returns richer result.
• packages/stellar-kit/test/readiness.test.ts (new) — 11 tests: ready, warnings (same src/dest), blocked (bad asset/amount), unsafe-network (mainnet disabled), unfunded, insufficient-funds, plus helper tests for getReadinessState/mapReadinessToErrorCode.
• examples/payment-readiness.example.json — deterministic intent + expected state/stages.
• apps/web/app/payments/page.tsx — readiness panel now shows the typed state badge (Ready / Ready with warnings / Unsafe network / Blocked) and a per-stage status grid; submission stays disabled unless state is ready/warnings.
• docs/transaction-readiness.md — engine reference, lifecycle, stages, API, safety notes.

Verification

• packages/stellar-kit full suite: 149 tests passing (11 new for readiness).
• packages/stellar-kit builds; web app typechecks clean.

Acceptance criteria

• Transaction readiness engine implemented (typed, cross-package).
• Account, asset, amount, memo, and network checks included (plus balance/funding).
• Readiness states are typed (ReadinessState).
• Web payment page uses the shared engine (badge + stage grid).
• Tests cover valid, invalid, unfunded, and unsafe-network scenarios.
• Documentation explains the readiness lifecycle.

Notes

• The engine only validates; it never builds or submits a transaction.
• Mainnet is disabled by default → MAINNET_DISABLED warning yields unsafe-network state.