1.1.0-alpha2: skip admin routes; substitute log placeholders inline

Two bug fixes from the alpha1 field test on portal-dev. The cache-
poisoning canary (3 cycles x 3 page shapes x 2 users = 18 trials)
passed cleanly on alpha1, validating the architecture; what surfaced
was scope-creep on the private-cache emission rather than a
correctness flaw.

Fix 1: admin routes no longer emit any LSCache header. Alpha1's
decision tree only checked Cache-Control: no-store to disqualify
responses, but admin pages return must-revalidate, no-cache, private
(same as authenticated content) and so were being held in private
cache for the full TTL. Practical impact was bounded by per-user
keying (no cross-user leakage) but settings forms looked stale to
operators for up to 600s after a change. Switched to AdminContext
::isAdminRoute() for the skip - covers everything under /admin/*
plus contrib routes flagged _admin_route: TRUE.

Fix 2: debug log messages now contain resolved values rather than
literal @tags / @ttl placeholder text. Drupal's dblog UI substitutes
the placeholders correctly, but drush watchdog:show doesn't run the
same pipeline. Alpha1 used the deferred-substitution pattern; alpha2
uses inline sprintf strings so every log consumer sees resolved
values.