refactor(pxe): separate the operation queue from staged-write jobs (backport of aztec-labs-eng/aztec-node#20)

[nventuro]

Backport of aztec-labs-eng/aztec-node#20 to the v5 line: a git cherry-pick -x of its squash commit on aztec-node/main, 10ab86541ef48443f2e1d53d2882acbf619278a1. The change itself was described and reviewed there; this PR is only about the port being faithful, so please review it as such.

Validating the backport

1. Fetch the source repo once: git remote add node https://github.com/aztec-labs-eng/aztec-node && git fetch node main.

2. Compare the upstream patch with this PR's patch (range-diff compares patches, so unrelated histories are fine):

   git range-diff 10ab86541e^..10ab86541e origin/nico/port-node-20-operation-queue^..origin/nico/port-node-20-operation-queue
   

   Expected: only the (cherry picked from commit …) trailer under the commit message and nothing else — no -/+ lines under any file header. The patch applied without conflicts and was not edited.

3. Build and run the unit suites at this commit (from yarn-project/): yarn workspace @aztec/pxe build && yarn workspace @aztec/txe build && yarn workspace @aztec/pxe test && yarn workspace @aztec/txe test. The upstream PR's own tests are part of these suites, so passing them is the behavioral check; CI on this PR runs them as well.

Stack

First PR of the stack, targets merge-train/fairies-v5 directly. Order: #20 → #35 → #40 → #47 → #94 → #100 → #98 (aztec-node numbers).