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.toml

@@ -19,6 +19,7 @@ members = [
     "crates/aura-registry",
     "crates/aura-research",
     "crates/aura-campaign",
+    "crates/aura-bench",
 ]
 
 [workspace.package]