install: the stable fallback is dead now that stable publishes its own bundles

It existed for one window: `stable` was a manifest-only pointer at whatever `v*`
tag had last been cut, and `stable` is the DEFAULT channel, so without the
fallback `curl … | sh` was broken for everyone between step 3 landing and the
first merge to `main`. That merge happened (`b6673c6`), and `stable` now holds
its own signed bundles at 1.0.3503145 — AppImage, deb and pacman, all resolving
by the one lookup both channels share.

Kept as a fallback it stops being a safety net and becomes a mask: the branch
only runs when `stable` has no bundles, which from here on means something is
broken, and chasing a `v*` release instead of saying so is the wrong answer.

The header now says the transition is finished and that neither channel should
be special-cased again, because the shape of that code invites re-adding it.

desktop/packaging/install.sh

@@ -10,7 +10,10 @@
 # Both are fixed-tag releases: the tag never moves and the assets are pruned to the
 # current build, so the tag alone names the newest one. `stable` only became one in
 # M314 step 3, when `main` started publishing — before that it was a manifest-only
-# pointer at whatever `v*` tag somebody had last cut.
+# pointer at whatever `v*` tag somebody had last cut, and this script carried a
+# fallback that chased the `v*` release its manifest named. That came out once
+# `main` had published to `stable` for real (`b6673c6`); the two channels are the
+# same shape now and nothing here should special-case one of them again.
 # Pick one with `--channel dev` or `TS_CHANNEL=dev`. Through a pipe the options go
 # after a `--`:  curl -fsSL <url> | sh -s -- --channel dev
 #
@@ -86,36 +89,13 @@ esac
 # --- resolve the release for this channel -----------------------------------
 say "Finding the latest ThoughtSync build on the $channel channel…"
 
-# ONE lookup for both channels now. Each is a release whose tag never moves and whose
-# assets are pruned to the current build, so the tag alone names the newest build on
-# that channel — which is exactly what an installer wants and what the in-app updater
+# ONE lookup, both channels. Each is a release whose tag never moves and whose assets
+# are pruned to the current build, so the tag alone names the newest build on that
+# channel — which is exactly what an installer wants and what the in-app updater
 # already reads.
 json="$(curl -fsSL "$API/releases/tags/$channel" 2>/dev/null)" ||
   die "the $channel channel has nothing published yet."
 
-# TRANSITIONAL — delete with the rest of the old scheme (M314 step 7).
-#
-# `stable` existed before this as a manifest-ONLY pointer: `latest.json` naming a
-# version whose bundles lived on a separate `v<version>` release. Between this commit
-# and the first merge to `main` it still looks like that, and `stable` is the DEFAULT
-# channel — so without this fallback `curl … | sh` is broken for everyone in that
-# window. It costs nothing once main has published: the grep finds the bundles and
-# this branch never runs again.
-if [ "$channel" = "stable" ] && ! printf '%s' "$json" | grep -q "releases/download/stable/[^\"]*\.\(AppImage\|deb\|pkg\.tar\)"; then
-  say "stable has no bundles of its own yet — falling back to the version its manifest names."
-  manifest="$(curl -fsSL "$INSTANCE/$REPO/releases/download/stable/latest.json" 2>/dev/null || true)"
-  stable_version="$(printf '%s' "$manifest" |
-    grep -oE '"version"[[:space:]]*:[[:space:]]*"[^"]+"' | head -1 |
-    sed -E 's/.*"([^"]+)"$/\1/')"
-  if [ -n "$stable_version" ]; then
-    json="$(curl -fsSL "$API/releases/tags/v$stable_version" 2>/dev/null)" ||
-      die "the stable channel names $stable_version, but there is no v$stable_version release to install."
-  else
-    json="$(curl -fsSL "$API/releases/latest" 2>/dev/null)" ||
-      die "no stable build published yet — try --channel dev, or merge to main."
-  fi
-fi
-
 # Pull asset URLs straight out of the release JSON (no jq). Anchored on the closing
 # quote so a `…AppImage.sig` URL can't be truncated into a match of its own.
 asset_url() {