1.3.0-alpha2: forward-port of 1.1.0-alpha2 fixes onto vary-cookie baseline

First public alpha of the 1.3.x track. Same vary-cookie response
subscriber additions and config schema as the never-tagged
1.3.0-alpha1, plus the two response-subscriber correctness fixes
from 1.1.0-alpha2 brought forward via merge of 1.2.x (which itself
merged 1.1.x):

- Admin routes no longer emit any LSCache header.
- Debug log messages contain resolved values inline.

1.3.0-alpha1 was implemented and CI-validated but never tagged or
released, since the alpha1 field-test loop ran on 1.1.x first.
1.3.0-alpha2 is the first public alpha of the 1.3.x track.