refactor: encode MorphTx version as prefix byte instead of RLP field

[FletcherMan]

1. Purpose or design rationale of this PR

Optimizes MorphTx encoding by moving the Version field outside of the RLP structure and encoding it as a prefix byte, similar to how Transaction.Type is handled.

Changes

Encoding Format

• V0 (legacy): 0x7F + RLP([chainID, nonce, ..., feeLimit, v, r, s])
  • Maintains full backward compatibility with existing AltFeeTx data
  • First inner byte is RLP list prefix (>= 0xC0)
• V1 (new): 0x7F + 0x01 + RLP([chainID, nonce, ..., feeLimit, reference, memo, v, r, s])
  • Version byte (0x01) precedes the RLP data
  • Enables future version upgrades without RLP structure changes

Version Detection Logic

firstByte == 0x00 || firstByte >= 0xC0 → V0 format (legacy)
firstByte == 0x01                      → V1 format
other                                  → unsupported version error 

Files Changed

• core/types/morph_tx.go - Updated encode()/decode() logic
• core/types/morph_tx_compat_test.go - New compatibility test file

2. PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• build: Changes that affect the build system or external dependencies (example scopes: yarn, eslint, typescript)
• ci: Changes to our CI configuration files and scripts (example scopes: vercel, github, cypress)
• docs: Documentation-only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that doesn't fix a bug, or add a feature, or improves performance
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• test: Adding missing tests or correcting existing tests

3. Deployment tag versioning

Has the version in params/version.go been updated?

• This PR doesn't involve a new deployment, git tag, docker image tag, and it doesn't affect traces
• Yes

4. Breaking change label

Does this PR have the breaking-change label?

• This PR is not a breaking change
• Yes

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch morph-tx-encoding-optimize

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