`gateway.allow_all_users: true` (and the top-level spelling) in config.yaml
was a silent no-op: `_TOPLEVEL_BRIDGE` never forwarded it, GatewayConfig has
no field, and every allow-all reader (authz mixin default-deny branch,
startup access check, own-policy adapters, Discord/Matrix/Email plugin
gates) consults the GATEWAY_ALLOW_ALL_USERS env var only.
Bridge the YAML key into that env var in `bridge_core_env_settings`, the
one seam every reader already shares, instead of threading a new config
attribute through ten readers:
- first-writer-wins: an explicit env var beats YAML (matches every other
{PLATFORM}_* gate);
- skipped inside a multiplexed secondary profile's scope (#80099 class):
the secondary's config.yaml must not become the default profile's policy,
and the isolation test now asserts GATEWAY_ALLOW_ALL_USERS stays unset;
- a startup warning names config.yaml as the grant source, because the key
was inert until now and a forgotten `true` flips the posture to open.
Tests: both spellings authorize a stranger through `_is_user_authorized`;
`false`, absent key, and env=false over YAML=true all stay denied.
Docs: security guide, env-var reference, gateway internals.
Fixes#110690
5083d5f78e