perf(runtime): group queued mutations behind one durable root

[forhappy]

Object-only writes previously captured and published a durable root for every command. Group up to four already-queued mutations per Cell into one transaction and publication, while preserving individual request identities, outcomes, and commit sequences. Successful replies still require the exact durable root covering each command; reads and other barriers retain FIFO ordering.

Release completed request slots before reply delivery to prevent immediate follow-up reads from receiving capacity refusals. Overlap independent LTX work and refill bounded compaction transfers as sources finish, preserving descriptor/error order, fencing, recovery checks, and resource limits. Update the routing test observer to validate grouped publication ranges.

Validation:

• Workspace/MSRV, object/follower qualification, both routing modes, and cookbook CI pass on implementation head 98ba666.
• Regression tests cover durable reply gating, grouped outcomes, mailbox handoff, bounded transfers, cleanup, and cold recovery.

Performance reports and versioned datasets, including regressions and measurement limitations, are tracked in the performance folder.