The proxy previously had no spoofing defence on `X-Forwarded-*`
headers. Any client could forge `X-Forwarded-For`, `-Proto`, or
`-Host` and rotate rate-limit buckets / poison logs / mis-attribute
auth. F4 adds an explicit gateway-allowlist gate.
* New `headroom/proxy/forwarded_headers.py` module:
- `resolve_client_ip(request)` — IP for log/auth/rate-limit.
- `trusted_forwarded_headers(request)` — sanitised `{for,proto,host}`
dict (empty strings when the gate fails).
- Configured by `HEADROOM_PROXY_TRUSTED_GATEWAY_CIDRS`
(comma-separated CIDRs). Empty/unset = strict-secure default.
- Malformed CIDR raises `ValueError` (loud, no silent skip).
- Every spoof rejection emits `forwarded_headers_rejected` log
event with `peer_ip` + the three header values.
- Result cached on `request.state.{client_ip,forwarded}` so the
helpers run once per request.
* Anthropic handler now keys the rate-limit bucket on the gated IP.
* 36 tests cover: default-strict / allow-listed peer / non-allow-listed
peer (with log assertion) / IPv6 / IPv4-mapped IPv6 / multi-CIDR /
whitespace tolerance / malformed CIDR loud-fail / empty headers /
caching / FastAPI integration.
