D-135 amendment: DC1 interim apt caching proxy (dc-cache-proxy.sh) ...

Operator-ruled 2026-07-24 (GA-R5, utterance quoted in D-135): DC1 gets an
interim apt caching proxy (apt-cacher-ng) to unblock its build without waiting
on the full ~950 GiB debmirror; DC0 stays the full-mirror deployment under test;
DC1 proxy is replaced by a full mirror afterward. Both artifact-delivery models
thus get exercised this rehearsal (Roosevelt input, D-132/D-133).

- scripts/dc-cache-proxy.sh: site-keyed check/install; apt-cacher-ng proxy mode
  at the D-134 utility .4:3142; reuses the <site>-mirror-net layer (fails loud if
  absent); caches archive/security/UCA, consumed via juju apt-http-proxy.
- tests/dc-cache-proxy/run-tests.sh: 15 static cases, 15/15 PASS.
- D-135 AMENDED (design-decisions.md) + CURRENT-STATE Stage-4 DC0/DC1 split.
- DC1 debmirror PAUSED at ~330 GiB (kept dormant as fallback); measured that the
  vcloud uplink was NOT the shared bottleneck (pausing dc1 did not speed dc0).

Feasibility recon'd read-only first (apt-cacher-ng installable, upstreams 200 via
edge, port free). Live install on the DC1 rack is the separately-gated mutation.
repo-lint 0 fail; gauntlet ALL GREEN (78).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VUjEb7onHpdqFHUnio6iVw