Fix Solana API README: dev script path does not exist

[Abhishek-singh88]

Problem

app/solana/cmd/api/README.md references a dev script that does not exist: ./project/solana/scripts/dev.sh start

Evidence

• README line: ./project/solana/scripts/dev.sh start
• No dev.sh exists anywhere in the repo.

Impact

Onboarding instructions are broken; new contributors can’t start the server using the documented command.

Proposed Fix

Update the README to point to valid commands, e.g.:

• cargo run -p protochain-solana-api
• or existing scripts under scripts/tests/* if those are intended.

Acceptance Criteria

• README no longer references missing dev.sh
• Instructions use an existing, verified command