Add REMOTE_USER env var with four modes:
- session (default): sends session id, backward-compatible
- static: sends a fixed string (REMOTE_USER_STATIC)
- mapped: looks up session id in REMOTE_USER_MAP
- none: omits the header entirely
New RemoteUserMode enum, ConfigBag parsing/validation, and
StringTrait::authSuccessResponse resolves the header value based
on the configured mode. AcceptListener now receives ConfigBag as
a constructor dependency.
Addresses design consideration 1.2 (Remote-User header value is
user-controlled) from DESIGN_CONSIDERATIONS.md.
241 tests pass, 0 cs-fixer violations.
