refactor(pxe): migrate CapsuleStore to BaseStagingStore (backport of aztec-labs-eng/aztec-node#101)

[nventuro]

Backport of aztec-labs-eng/aztec-node#101 to the v5 line: a git cherry-pick -x of its squash commit on aztec-node/main, ff9545903b2592201515ff7d3af9e92587f15b6e. 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 ff9545903b^..ff9545903b origin/nico/port-node-101-capsule-store-staging^..origin/nico/port-node-101-capsule-store-staging
   

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

   -+import { BufferReader, serializeToBuffer } from '@aztec-labs/foundation/serialize';
   ++import { BufferReader, serializeToBuffer } from '@aztec/foundation/serialize';
   

   Same added import on both sides; the scope differs because upstream renamed its packages to @aztec-labs/* (chore: publish labs packages under the @aztec-labs npm scope aztec-labs-eng/aztec-node#93) and this repo keeps @aztec/*. Surrounding import lines show as context-only differences (single -/+ followed by a space) for the same reason. 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 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#98 (branch nico/port-node-98-fact-store-staging, PR #25368); 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).

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/aea6d638dcea0291�aea6d638dcea02918;;�): yarn-project/kv-store/scripts/run_test.sh src/bench/sqlite-opfs-encrypted/map_bench.test.ts (1s) (code: 0)