feat(compact-basic): baseline compaction backend (squashed from compact-basic)

Collapses the per-round review churn of the prior compact-basic branch into a
single clean baseline on top of compact-interface, so the upcoming retention
refactor lands as fresh, well-scoped commits rather than stacking on a history
of fixes that are being superseded.

packages/support/invariants/src/index.ts

@@ -162,9 +162,6 @@ function checkEvent(trace: SessionTrace, event: SessionEvent): void {
       trace.surface.push(event.seq)
     } else {
       const { start, end } = se.surfaceOp
-      if (start > end) {
-        throw new InvariantError(`surface replace: start ${start} must be <= end ${end}`)
-      }
       const startIdx = trace.surface.indexOf(start)
       if (startIdx === -1) {
         throw new InvariantError(`surface replace: start seq ${start} is not on the surface`)