ci: pin the build clock to the commit, so an unchanged refresh publishes nothing

Milestone 362 step 1, closing #3265's root cause.

The weekly base refresh rewrote all three `:latest` tags on 2026-08-30 with
nothing changed in any of them. Not a cache miss — run 4934's log shows every
content step CACHED and both bases resolved to unchanged pinned digests.
buildkit stamps the image config with the wall clock of the build, so
identical layers get republished under a new config blob and therefore a new
manifest digest.

The cost is not storage, it is meaning: `:latest` moved on a calendar, so a
digest change stopped being evidence that anything was different. That is the
one thing a digest is any use for, and it is load-bearing here — the reuse
check, the `:c-<sha>` rollback story and any future redeploy signal all rest
on it.

SOURCE_DATE_EPOCH normalises `created` and the history timestamps, so the same
source produces the same config bytes and the same digest, and pushing it is a
registry no-op.

The value is routed through artifacts.sh's existing `newest()` rather than
taken from git separately. `revision`, `version` and now `epoch` are three
fields of ONE lookup, so they cannot drift into naming different commits — a
divergence that would stamp an image reproducibly against one commit while it
reported being another, with both values looking perfectly well-formed. Note
#3127 §2 is the record of what a second clock costs; this adds a view, not a
clock.

Also corrected: the build step comment and ci-requirements.md both described
the churn as current behaviour with the fix as a "likely" future. They now
describe what the file does.

Tests pin the property the fix depends on, not the fix: epoch is the same
commit version names, in both renderings including the extension's unpadded
one, and it does not move between two calls on one checkout. A future
refactor that gave epoch its own `git log` would pass every other test in
that file.

Not yet verified end to end — proving it needs two consecutive refreshes to
land on the same digest, which is the next thing, and is the step #3265 exists
because nobody did last time.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TTjbZZ6JirCMSaJzQV1RhA

ci-requirements.md

@@ -198,14 +198,21 @@ per `docs/process.md`'s "add deps to the image when used by >1 project".
     refresh from being undone.
   - **`pull: true` on the scheduled path only** is the mechanism: a moved base
     tag changes the `FROM` layer's cache key and everything above it rebuilds.
-    **It does not currently make the unmoved case free.** Measured on the first
-    real fire (run 4934, 2026-08-30): every content step reported `CACHED` and
-    the bases resolved to unchanged digests, yet all three `:latest` tags got a
-    NEW manifest digest, because buildkit mints a fresh image config per run and
-    republishes identical layers under it. So `:latest` is rewritten weekly
-    whether or not anything changed, and `:c-<sha>` is handed a new manifest to
-    diverge from on the same cadence — a digest change stops meaning anything.
-    Tracked as #3265; the likely fix is a deterministic `SOURCE_DATE_EPOCH`.
+    It did not always make the unmoved case free. Measured on the first real
+    fire (run 4934, 2026-08-30): every content step reported `CACHED` and the
+    bases resolved to unchanged digests, yet all three `:latest` tags got a NEW
+    manifest digest, because buildkit stamps a fresh image config per run and
+    republishes identical layers under it — so `:latest` was rewritten weekly
+    whether or not anything changed, and a digest change stopped meaning
+    anything (#3265).
+  - **`SOURCE_DATE_EPOCH` is what makes it free.** Set on each build step from
+    `artifacts.sh epoch <artifact>` — the unix timestamp of the same commit
+    `revision` and `version` name, so all three are views of one `newest()`
+    lookup and cannot drift into disagreeing. With the config's `created` field
+    and history timestamps pinned to the content rather than to the wall clock,
+    identical source produces an identical manifest digest and the push is a
+    registry no-op. That restores the property the whole scheme rests on: a
+    channel tag's digest changes when, and only when, its content does.
     Separately not caught: a Debian package update inside the `apt-get install`
     layer while the base tag stands still — a lag rather than a hole, since the
     official python/cuda images rebuild with those updates baked in.