build: zero-pad the derived version to YYYY.MM.DD.HHMM (318 step 5)

`2026.8.28.1249` becomes `2026.08.28.1249`. Note #3127 §1 and rule 148 both
specify the padded form.

The old reasoning was that each segment should read as a plain integer, and
it never held — comparison strips leading zeros on parse anyway, which the
same paragraph said. What stripping actually bought was this project emitting
`2026.8.28.1432` while a sibling emitted `2026.08.28.1432`: two shapes one
character apart, which is the hard kind of difference to notice. Two
obviously different formats would be safer than two nearly identical ones,
and identical is safer still.

Nothing already published is reordered: comparison is numeric per dot-segment,
so `08` and `8` are equal.

strip0 goes, and with it three of the four git calls per version — git's
format-local takes the whole format string, and splitting it into pieces only
ever existed to strip the padding between them.

It also fixes a real edge the old helper mangled. A commit at 03:22 UTC
derived `322` for its HHMM field, silently turning a four-digit field into
three; it now derives `0322`. Verified against a real commit rather than
reasoned about.

Checked before relying on it, since step 8 feeds this to Firefox: the
extension's comparator is `parseInt(n, 10)` with an explicit radix, so `08`
reads as 8 and there is no octal hazard (rule 150).

Two tests added. One pins the padded shape — the only thing keeping the
family's projects emitting one string is an assertion that they do. The other
asserts version and revision describe the same commit: they are derived
independently, and a divergence would mean an instance naming one commit
while carrying another's bytes, which is unfalsifiable from outside because
both values still look well-formed.

scripts/artifacts.sh

@@ -100,13 +100,6 @@ fmt() {
   (cd "$ROOT" && TZ=UTC git show -s --format=%cd --date="format-local:$2" "$1")
 }
 
-# Leading zeros stripped so every segment is a plain integer — some version
-# validators reject `08`, and a leading zero buys nothing. `0000` (midnight)
-# must survive as `0`, not as the empty string.
-strip0() {
-  printf '%s' "$1" | sed -e 's/^0*//' -e 's/^$/0/'
-}
-
 # The IDENTITY of an artifact's content: the commit its shipped files last
 # changed in. This is what decides whether a build can be skipped.
 #
@@ -124,17 +117,32 @@ cmd_revision() {
   echo "$(newest "$1")" | cut -d' ' -f2 | cut -c1-12
 }
 
-# The ORDERING KEY: full precision, YYYY.M.D.HHMM. Used by the extension,
-# where the value is what Firefox compares to decide whether an update exists
-# — two same-day builds MUST be distinguishable or the second never reaches
-# anyone.
+# The VERSION: `YYYY.MM.DD.HHMM`, zero-padded, UTC. One shape across the whole
+# family (note #3127 §1, rule 148) — the number an instance reports about
+# itself, and, with a `v` in front, the release tag naming the same build.
+#
+# Zero-padded since 2026-08-28. This stripped leading zeros until then, on the
+# reasoning that every segment should read as a plain integer — which never
+# held, since comparison strips them on parse anyway. Padding costs nothing,
+# sorts lexically as well as numerically, and keeps this project emitting the
+# same string as its siblings: unpadded, a `2026.8.28.1432` here sits beside a
+# `2026.08.28.1432` there, two shapes one character apart. Two obviously
+# different formats are safer than two nearly identical ones.
+#
+# Comparison is numeric per dot-segment, so `08` and `8` are equal and nothing
+# already published is reordered by the change.
+#
+# HHMM is not decoration: it is what makes the value unique per build with no
+# lookup. A date alone collides on the second build of a day, and resolving
+# that needs a `.N` suffix, which needs asking the registry what already
+# exists — at which point two lanes derive different answers for one source
+# and the shared-signature property is lost.
 cmd_version() {
   sha=$(echo "$(newest "$1")" | cut -d' ' -f2)
-  printf '%s.%s.%s.%s\n' \
-    "$(fmt "$sha" %Y)" \
-    "$(strip0 "$(fmt "$sha" %m)")" \
-    "$(strip0 "$(fmt "$sha" %d)")" \
-    "$(strip0 "$(fmt "$sha" %H%M)")"
+  # One git call for the whole string rather than four and a sed. git's
+  # format-local takes the complete format, and doing it in pieces was only
+  # ever there to strip the padding between them.
+  fmt "$sha" '%Y.%m.%d.%H%M'
 }
 
 [ $# -ge 2 ] || usage