[PeerX Backlog # 83] Atomic batch settlement with delta snapshots (drop the "Layer-2" framing)

[ameeribro4-sudo]

Problem Statement
A user submitting N swaps in an atomic batch currently pays roughly
N× the per-swap cost because every swap re-runs KYC, fee math,
cache invalidation, and event emission. The "atomic" semantic requires
all-or-nothing rollback, which today is implemented by cloning the
entire Portfolio (see Issue #70).

(The historical title called this "Layer-2" — that framing implied
rollup / optimistic semantics that do not apply here. Retitled.)

Expected Outcome
Build on Issue #70's delta snapshot machinery so that pre-settlement
state can be cheaply rebuilt, and amortize the fixed costs of KYC,
rate limits, cache invalidation, and event emission across N swaps in
a single batch.

Acceptance Criteria

• Established by measurement: run the existing execute_batch_atomic
  on a 100-swap fixture and capture baseline gas; the implementation
  target is "≤ baseline minus amortized fixed-cost overhead" — i.e.
  no worse than the current implementation, with N swaps paying one
  KYC check, one cache flush, and one batched event.
• State is recoverable on partial failure via the delta list (Issue [PeerX Backlog # 70] Avoid cloning entire Portfolio for atomic batch #70).
• No regressions in the 100% rollback-on-failure property test.

Files Affected

• peerx-contracts/counter/src/lib.rs, batch.rs.

Dependency Issue #70.
Difficulty / Effort 🥑 / XL
Labels feature, performance, priority/medium

[GBOYEE]

Hi team — I'd like to take this on as part of the GrantFox campaign. I'll implement it and open a PR referencing this issue shortly.