feat(solana/session): tightening operator-signed mode, add idle timeout

[lgalabru]

Summary

• add negotiated idle-timeout handling and server-initiated idle close
• distinguish client- and operator-signed voucher modes with voucherSigner
• define a payer-bound reusable session access proof and atomic operator metering

Base cleanup

• rebase onto current Tempo main
• exclude operational-cost changes already merged in feat(solana/sessions): operational costs reduction #296
• limit the PR to draft-solana-session-00.md

Test Plan

• render with kramdown-rfc and xml2rfc
• run rfclint on the generated XML
• run git diff --check

AI Assistance

Codex assisted with rebasing the branch, narrowing the diff, editing the draft, and validation.

[brendanjryan]

👍

[brendanjryan]

@lgalabru -- some CI issues but otherwise looks good

[lgalabru]

Thanks for the review @brendanjryan, CI fixed!