feat(bench): scaffold aura-bench — baseline compare core + synthetic data plumbing

Tasks 1-2 of the bench-harness plan (refs #251): the new dev-only workspace
bin crate with the report-only comparison core (BaselineDoc serde, per-metric
relative drift, 10% NOTICE threshold, fingerprint-equality exit code) and the
deterministic synthetic inputs (seeded LCG price walk, the proven 48-byte
Delphi-record M1 zip archive writer, self-deleting scratch dirs).

The archive writer is deliberately a third copy of the test-fixture packer
(aura-cli tests / aura-ingest each carry one), byte-compatible by
construction and pinned here by a round-trip test through the real ingest
reader. The binary refuses debug builds (exit 2) — wall-clock numbers from a
debug profile measure the profile, not the code — pinned by an E2E spawn
test. Temporary #![allow(dead_code)] markers cover the not-yet-wired core
until the CLI glue lands; the surfaces and driver follow.

Cargo.lock

@@ -105,6 +105,20 @@ dependencies = [
  "serde_json",
 ]
 
+[[package]]
+name = "aura-bench"
+version = "0.1.0"
+dependencies = [
+ "aura-core",
+ "aura-engine",
+ "aura-ingest",
+ "aura-std",
+ "clap",
+ "serde",
+ "serde_json",
+ "zip",
+]
+
 [[package]]
 name = "aura-campaign"
 version = "0.1.0"