docs: amend architecture per migration reverse constraints
Resolve 13 gaps flagged by the three migration documents: finalize
chat() signature with per-call cache namespace/salt, fix middleware
onion ordering (per-attempt source-scoped admission inside retry
loop), structured AllSourcesExhausted fields, est_tokens in
SourceConfig, per-scope resilience config keys, probe lease TTL,
settle/release idempotency, shared limiter/breaker backend semantics,
multimodal digest in telemetry, OCR trust_env and health check, API
stability rules, and correct the Q3 embedding claim.
iomgaa
07e772f522