perf(continuations): Emit COMMIT output on the Memory bus

[nicole-graus]

Motivation

In continuations, COMMIT output was verified per epoch, needing extra logic to check the right substring of committed bytes and a driver-trusted concatenation of the per-epoch output slices. COMMIT output is really a global property.

What changed

• The COMMIT chip emits its committed bytes on the shared Memory bus under the existing domain separator (renamed is_register → domain, now 0 RAM / 1 register / 2 commit) instead of a dedicated Commit bus.
• Continuations verify the output once, globally: each epoch's COMMIT trace is re-committed in the global proof and root-bound to the epoch proof, and the whole run's output is balanced there and absorbed into the global statement. The per-epoch commit offset and the untrusted slice concatenation are removed, so the aggregate output is verifier-checked.
• domain is pinned to {0,1} with IS_BIT in MEMW/MEMW_ALIGNED so the commit domain stays disjoint.