chore(release): name the 2.1 line twinned (#453)

* chore(release): name the 2.1 line twinned

The codename had read "navigable" since the 1.9 line and went out
unchanged with 2.0. Nothing derives it from the release and no guard
covers it — the sibling `version` line got one after the hero drifted to
a stale number, but the name beside it did not.

"twinned" is the release: one session, two outputs with the same
geometry.

The file's own comment claimed the release script stamps the banner with
both values. It stamps only the version — neither banner renderer reads
the codename. It is read by EngineDeckExample alone, whose committed
asset nothing regenerates, which is why a stale name survives a release
unnoticed. The comment now says that.

* test(examples): update the engine-deck layout baseline for the new codename

The deck renders the codename as a text node, so renaming the release
moves its measured width: "navigable" is 59.92 pt in Helvetica-Oblique at
14 pt, "twinned" is 48.244 pt. Those two values are the whole baseline
delta — the tag sits in a fixed-width version block, so nothing
downstream shifts.

Author: DemchaAV

examples/src/main/resources/banner.properties

@@ -3,8 +3,15 @@
 # `version` is filled by Maven resource filtering from the reactor
 # ${project.version} at build time (see examples/pom.xml <resources>), so the
 # rendered hero is never a hand-bumped literal and can never drift from the
-# release it was cut from. `codename` is the per-minor release name, set during
-# release prep. cut-release.ps1 re-renders the banner on every tag, stamping the
-# committed assets/readme/repository_showcase_render.png with both values.
+# release it was cut from. cut-release.ps1 re-renders
+# assets/readme/repository_showcase_render.png on every tag so that hero always
+# carries the version it was cut from.
+#
+# `codename` is the per-minor release name and is NOT derived from anything —
+# update it by hand during release prep. It is read only by EngineDeckExample
+# (title, section header, version block, and the footer on every slide); the
+# hero banner renderers do not use it. Nothing regenerates that deck's committed
+# asset, so a stale codename survives a release unnoticed — which is how it sat
+# on the 1.9 name through 2.0.
 version=@project.version@
-codename=navigable
+codename=twinned

examples/src/test/resources/layout-snapshots/flagships/engine-deck.json

@@ -895,11 +895,11 @@
     "computedY" : 503.938,
     "placementX" : 570.0,
     "placementY" : 503.938,
-    "placementWidth" : 59.92,
+    "placementWidth" : 48.244,
     "placementHeight" : 12.95,
     "startPage" : 0,
     "endPage" : 0,
-    "contentWidth" : 59.92,
+    "contentWidth" : 48.244,
     "contentHeight" : 12.95,
     "margin" : {
       "top" : 0.0,