Build npm Package: @near-js/tx-builder - Transaction Construction

Description

## Overview
Create an npm package for building complex NEAR transactions with a fluent API.

## Why This Matters
Complex transactions are error-prone. A builder pattern makes construction safe and readable.

## Technical Requirements

### Builder API
```typescript
import { TransactionBuilder } from '@near-js/tx-builder';

const tx = new TransactionBuilder()
.setSigner('alice.near')
.setReceiver('contract.near')
.addAction('functionCall', {
methodName: 'transfer',
args: { to: 'bob.near', amount: '1000000' },
gas: '30000000000000',
deposit: '1'
})
.addAction('transfer', { amount: '1000000000000' })
.build();
```

## Deliverables
1. **Transaction builder package**
2. **Fluent API**
3. **All action types**
4. **Validation built-in**

## Budget: 8 NEAR

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