fix: correct README examples, signer doc.go, and Validate nil checks

[giwaov]

Summary

• Fix 5 non-compiling README code examples — client.New() returns a single value (not a tuple), SendTransaction(tx) doesn't exist (must use Serialize() + SendRawTransaction()), and hash.Hex() is called on a string
• Fix signer/doc.go example — Sign() takes common.Hash, not []byte; the example should use SignData() instead
• Add nil checks in Validate() for MaxFeePerGas and MaxPriorityFeePerGas — prevents nil pointer panics in Clone() and Serialize()
• Add missing keychain package to the Packages documentation table
• Fix "crates" → "packages" terminology (Go, not Rust)

Detailed Changes

README.md

Line(s)	Issue	Fix
Quick Start	c, _ := client.New(...) won't compile — New() returns *Client	c := client.New(...)
Quick Start	c.SendTransaction(tx) doesn't exist	transaction.Serialize(tx, nil) + c.SendRawTransaction(ctx, serialized)
Quick Start	hash.Hex() called on string	hash (already a string)
Quick Start	Missing context import	Added "context"
Quick Start	Uses RpcUrlMainnet	Changed to RpcUrlModerato (testnet example)
4 examples	client.SendTransaction(tx)	Same Serialize + SendRawTransaction pattern
Packages table	Missing keychain package	Added row
License	"crates" (Rust term)	"packages" (Go term)

pkg/signer/doc.go

• signer.Sign(data) where data is []byte → signer.SignData(data) (correct method for []byte input)

pkg/transaction/transaction.go

• Added nil checks for MaxFeePerGas and MaxPriorityFeePerGas in Validate(), matching the existing pattern for ChainID and NonceKey nil checks

Test plan

• Verify Quick Start example compiles with go build
• Verify Validate() returns error for nil MaxFeePerGas/MaxPriorityFeePerGas
• Run existing test suite: make test
• Verify go doc github.com/tempoxyz/tempo-go/pkg/signer shows correct example