refactor(pxe): migrate FactStore to BaseStagingStore (backport of aztec-labs-eng/aztec-node#98)

[nventuro]

Backport of aztec-labs-eng/aztec-node#98 to the v5 line: a git cherry-pick -x of its squash commit on aztec-node/main, 4a7a2e233528f717e6c22e894526831367d80be3. 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 4a7a2e2335^..4a7a2e2335 origin/nico/port-node-98-fact-store-staging^..origin/nico/port-node-98-fact-store-staging
   

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

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

   Same removal on both sides; the package scope differs because upstream renamed its packages from @aztec/* to @aztec-labs/* (chore: publish labs packages under the @aztec-labs npm scope aztec-labs-eng/aztec-node#93) before this PR merged there, and this repo keeps @aztec/*. For the same reason the surrounding import lines show up as context-only differences (a single - or + followed by a space). No other file differs.

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 the upstream scope rename leaked nothing into this repo: git grep -c @aztec-labs -- yarn-project on this PR's head must return no matches.

Stack

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