The previous round (88f0923) threaded a `backend_id` from `BACKEND_ID`/`HOSTNAME`
env vars into `AppState`. That required the operator to manually align each
instance's `BACKEND_ID` with the Cloudflare Worker's `BACKEND_ORIGINS_JSON`, which
is exactly the kind of manual cross-config sync that drifts.
The request's own `Host` header already carries the answer: the Worker load
balancer (`ARCH-LB-WORKER`) preserves the inbound `Host` while rewriting the target
to a backend origin (`joinUrl` builds `origin + path + query`), so `Host` is the
public address the LB routed this instance as. `backend_label_from_headers` reads
it per-request in `setup_status`, trims the default https/http port
(`example.com:443` -> `example.com`) so the label mirrors a bare origin, and returns
`None` when the request carries no usable `Host`.
This removes the startup env entirely — `backend_id_from_env`, `AppState::backend_id`,
all five router-builder `backend_id` parameters, the `.env.example` `BACKEND_ID`
entry, and the `BACKEND_ID` env documentation are all gone. Handlers still never
touch the environment: `Host` is request-scoped, not process-global, so there is no
test race (the `debug_surface_requested` rationale was about env, not headers).
Tests: `setup_status_reports_backend_label` now sends `Host: edge-1.example.com:443`
and asserts the label is the bare origin `edge-1.example.com` (port trimmed). A new
`setup_status_reports_null_backend_without_host` asserts `null` when no Host is sent.
GATE-P0 106 passed / 0 failed / 4 ignored; GATE-DOCS 6/6 clean (49 line-refs).
Sync the stated-as-current gate numbers that the previous feature commit
(88f0923) shifted, per the same-round rule:
- Test baseline 104 -> 105 passed / 0 failed / 4 ignored across the
authoritative (charter §6.1) and deferring docs (design §Phase 0,
opengaps access-control, HANDOFF-push-401), with retest date 2026-10-14.
The new test `setup_status_reports_backend_label` is a normal (non-ignored)
test, so the ignored set is unchanged.
- opengaps anchor count 72 -> 49, matching what `run_all.sh` ANCHOR AUDIT now
reports (the design.md main.rs line-anchors were converted to symbol refs).
- AGENTS version baseline 3b36fe3 -> 88f0923 (gate-holder merged this batch;
88f0923 is the gate-green commit of the round). Charter Code Baseline
`fc686ab` left as-is — line-number anchor semantics were not bulk-reverified
this round, so claiming otherwise would overstate (§7.4).
GATE-DOCS 6/6 clean (49 line-refs, 0 errors); GATE-P0 stood at 105/0/4.
anonsaber