fix(proxy): quarantine poisoned bridge anchors at circuit open - #50

Summary

• Quarantine hard HTTP bridge keys when poison-class eventless failures open the retry circuit.
• Keep quarantine through remaining cooldown plus half-open lease, including durable conflict merges.
• Suppress proxy anchor injection for verified full resends while preserving delta-only continuity.
• Record eventless terminal failures attempt-scoped, gate strikes on no-safe-replay, clear abandoned anchors, and settle only stranded circuits.

Verification

• uv run pytest -q tests/unit/test_proxy_http_bridge.py — 681 passed
• uv run pytest -q tests/integration/test_http_responses_bridge.py — 151 passed
• make lint — passed
• make typecheck — passed
• openspec validate recover-poisoned-anchor-on-circuit-open --strict — passed
• openspec validate --specs — 57 passed
• make ci-fast — backend gates passed, then stopped because bun is not installed (frontend-install: bun: command not found)

Source sidechat: 01a04ac6-27a1-74b3-88d3-f1ede3901184
Recovered from: 8c6ff6b