docs(design): shared io_uring instance across concurrent transfers (#1060)

[oferchen]

Summary

• Design note for Allow whitespace in bandwidth limit arguments #1060: shared io_uring instance across concurrent in-flight transfers in a single daemon.
• Cross-references the completed session-level ring pool (Add xtask docs command for documentation validation #1408/Document oc release branding and enforce README validation #1409) and positions this work as the next tier for high-connection-count daemons.
• Specifies an Arc<SharedIoUringRouter> with packed user_data completion routing, per-conn backpressure, fairness via token-bucket admission, and security via IORING_REGISTER_RESTRICTIONS.
• Recommends gating behind --features shared-io-uring and keeping per-session pool as the default.

Test plan

• Reviewer confirms the design distinguishes shared-across-sessions from the existing per-session pool.
• Reviewer agrees the trade-offs (routing, backpressure, fairness, security, blast radius) are addressed.
• Reviewer agrees the feature-gate rollout plan is conservative enough.