docs: record Symfony 8.1 upgrade results, update stack references

- Plan doc: mark Phases 0-3 + worker-mode state audit complete with
  verified results; resolve open questions (all src/ services are final
  readonly - no ResetInterface needed under kernel-reusing runner)
- CHANGELOG: Unreleased entries for the 8.1 bump and the
  runtime/frankenphp-symfony removal (incl. FRANKENPHP_LOOP_MAX note)
- ROADMAP: current stack now Symfony 8.1; webauthn research note updated
- bin/dev.sh: correct stale PHP requirement comment

bin/dev.sh

@@ -46,7 +46,7 @@ REQUIRED_PHP_EXTS=(
 )
 
 # Apt packages for PHP + extensions
-# Note: preauth uses Symfony 7.4 which requires PHP >=8.1.
+# Note: preauth uses Symfony 8.1 which requires PHP >=8.4.
 # We install PHP 8.4 (available in Debian 13/Trixie) for consistency.
 PHP_APT_PACKAGES=(
     php8.4-cli