feat: add --tx-version option to build legacy transactions

[bangyro]

Summary

Adds a global --tx-version <version> option to the CLI so users can build legacy transactions instead of version 0 transactions. Squads v3 only accepts legacy transactions when importing a base58 encoded transaction, so exports for a v3 multisig failed to import.

Usage

npx @solana-program/program-metadata@latest write idl <program-address> \
  --buffer <buffer-address> \
  --export <multisig-address> \
  --export-encoding base58 \
  --close-buffer <your-address> \
  --tx-version legacy

[lorisleiva]

@trevor-cortex

[trevor-cortex]

Summary

Adds a global --tx-version <version> CLI option (legacy | 0, default 0) so exported transactions can be built as legacy transactions — needed because Squads v3 only accepts legacy transactions when importing base58-encoded transactions. The option is threaded into the transaction planner via a new getTransactionConfig helper, and the README documents both the flag and the Squads v3 workflow.

The implementation looks solid:

• TransactionPlannerConfig in @solana/kit-plugin-rpc accepts version: 0 | 'legacy' (defaulting to 0), so passing the parsed value through is correct, and the explicit version: 0 default preserves the previous behavior.
• The .choices() + .default() + .argParser() pattern matches the existing house style in options.ts (compressionOption, encodingOption, etc.). Nice touch using the parsed representation (0 as a number) for the default value, since Commander doesn't run argParser on defaults — an easy trap to fall into.
• The custom InvalidArgumentError message mirrors Commander's own choices-validation wording, which matters since .argParser() replaces the validator that .choices() installs.

Notes for reviewers

• One non-blocking inline suggestion on getTransactionConfig: the exhaustive switch is a nice future-proofing pattern, but since this tsconfig doesn't enable noImplicitReturns, the compiler won't actually catch the missing case when a version 1 is added later — see inline comment.
• Out of scope for this PR, but adjacent to the new code path: priorityFeesOption uses .default('100000') with a string default, and since Commander doesn't run argParser on defaults, microLamportsPerComputeUnit receives the string '100000' (not a MicroLamports bigint) whenever the flag isn't passed. Pre-existing behavior, just worth a follow-up look.

[lorisleiva]

That's a great improvement and it'll make it easier for us to add tx v1 support so thank you!