refactor(pxe): migrate PrivateEventStore to BaseStagingStore (backport of aztec-labs-eng/aztec-node#110)

[nventuro]

Backport of aztec-labs-eng/aztec-node#110 to the v5 line: a git cherry-pick -x of its squash commit on aztec-node/main, 4dfe50f49caadc3cbd9b0178af19e7e4e7bd71e8. 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 4dfe50f49c^..4dfe50f49c origin/nico/port-node-110-private-event-store-staging^..origin/nico/port-node-110-private-event-store-staging
   

   Expected: the (cherry picked from commit …) trailer and exactly one patch-level difference, in yarn-project/pxe/src/storage/private_event_store/private_event_store.ts:

   --import { Semaphore } from '@aztec-labs/foundation/queue';
   +-import { Semaphore } from '@aztec/foundation/queue';
   

   Same removal on both sides, under this repo's @aztec/* scope instead of upstream's @aztec-labs/* (their refactor: removed duplicated fields and addresses #93 rename, not taken here). Two conflicts were resolved, both scope/context only: the import block above, and yarn-project/txe/src/oracle/interfaces.ts, where v5's getLastTxEffects keeps its inline return type (upstream has the TxEffectsData struct from a refactor that never landed on v5) while getPrivateEvents takes upstream's new changeSetId parameter verbatim.

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.

4. Confirm upstream's package-scope renames leaked nothing into this repo: git grep -cE '@aztec-labs|@aztec-foundation' -- yarn-project on this PR's head must return no matches.

Stack

Stacked on the backport of aztec-labs-eng/aztec-node#108 (branch nico/port-node-108-sender-tagging-staging); to be rebased onto merge-train/fairies-v5 once that one merges. Order: #20 → #35 → #40 → #47 → #94 → #100 → #98 → #101 → #108 → #110 (aztec-node numbers).