Tracking: net-http Tier-2 hardening (ADR-deferred)

[NotAProfDev]

Deferred hardening from docs/superpowers/plans/2026-07-06-net-http-deep-review.md §2B (each needs an ADR amendment; split into its own issue+PR when picked up):

• Rolling-window / error-rate circuit breaker (replace consecutive-count)
• Per-key circuit breakers
• Retry-After header honoring (503/429)
• Total-elapsed retry budget / retry-token bucket
• Request priority / fairness (orders preempt market data)
• Bounded parked-request backpressure signal to Core
• Outbound-proxy support (env proxy / HTTP CONNECT)
• Response decompression (gzip/deflate/br)
• Streaming stall TimeoutBody