6-layer architecture

Agentic SpendGuard organizes its concerns into 6 primitive layers, executed in this strict order on every decision:

T (Trace) → L (Ledger) → C (Contract) → D (Decision) → E (Evidence) → P (Proof)

Layer	Responsibility	Key invariant
T Trace	Capture event identity (run_id, step_id, llm_call_id)	Every event has a globally-unique id
L Ledger	Atomic budget reservation + commit	Per-unit balance preserved every tx
C Contract	Hot-path policy evaluation	Decision in <5ms
D Decision	8-stage transaction state machine	Stages 1-4 always atomic
E Evidence	Audit chain durability	No effect without audit row (§6.1)
P Proof	Per-event signing + verification	Cosign-signed bundles + Ed25519 events

See docs/contract-dsl-spec-v1alpha1.md and docs/stage2-poc-topology-spec-v1alpha1.md in the source repo for the full specifications.