1.2.0-alpha2: forward-port of 1.1.0-alpha2 fixes onto ESI baseline

First public alpha of the 1.2.x track. Same ESI render element and
fragment-route work as the unreleased 1.2.0-alpha1, plus the two
response-subscriber correctness fixes from 1.1.0-alpha2 brought
forward via merge of 1.1.x:

- Admin routes no longer emit any LSCache header (AdminContext-
  based skip, covers /admin/* plus contrib routes flagged
  _admin_route: TRUE).
- Debug log messages contain resolved values rather than literal
  @placeholder text (inline sprintf substitution, works in dblog
  UI and drush watchdog:show alike).

The 1.2.0-alpha1 tag was cut but never published as a release
node, since the alpha1 field-test loop ran on 1.1.x first and
surfaced bugs that needed to land before any 1.2 alpha hit
operators.