refactor: hoist platform_install_tier to core as apt-family default

ubuntu.sh and server.sh had near-identical platform_install_tier copies that
diverged only in whether the low tier installed snaps. That divergence was
incidental (no low-priority server snaps exist) and would have silently dropped
any future low server snap. Make the low tier run snap uniformly and hoist one
shared copy into lib/core.sh beside apt_install_tier; arch.sh keeps its yay
override. Package selection stays data-driven via package_manager.<platform> in
packages.json, so a manager only installs where a package declares it -- no
profile conditional needed in the install path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ulises-c

lib/core.sh

@@ -417,6 +417,30 @@ apt_install_tier() {
   run sudo apt install -y $names
 }
 
+# apt + snap tier composition — shared by the Ubuntu desktop and the server
+# profile. Selection is data-driven (package_manager.<platform> in packages.json),
+# so each manager runs at every tier uniformly; a package only installs where it
+# declares that platform. arch.sh overrides this with the yay batch.
+platform_install_tier() {
+  case "$1" in
+    high)
+      printf '==> Installing high-priority apt packages...\n'
+      apt_install_tier high
+      ;;
+    medium)
+      printf '==> Installing medium-priority apt packages...\n'
+      apt_install_tier medium
+      printf '\n==> Installing snap packages...\n'
+      snap_install_tier medium
+      setup_bat_fd_symlinks
+      ;;
+    low)
+      apt_install_tier low
+      snap_install_tier low
+      ;;
+  esac
+}
+
 # Tailscale via the official curl installer — apt-family default (ubuntu + server).
 # arch.sh overrides this (tailscale ships in the yay batch; just enable the daemon).
 platform_tailscale_step() {

platforms/server.sh

@@ -10,25 +10,6 @@ platform_bootstrap() {
   apt_bootstrap
 }
 
-platform_install_tier() {
-  case "$1" in
-    high)
-      printf '==> Installing high-priority apt packages...\n'
-      apt_install_tier high
-      ;;
-    medium)
-      printf '==> Installing medium-priority apt packages...\n'
-      apt_install_tier medium
-      printf '\n==> Installing snap packages...\n'
-      snap_install_tier medium
-      setup_bat_fd_symlinks
-      ;;
-    low)
-      apt_install_tier low
-      ;;
-  esac
-}
-
 # Pre-clone the antidote plugins now while the network is provably up; otherwise
 # the first interactive login does the GitHub clones lazily.
 server_preclone_antidote() {

platforms/ubuntu.sh

@@ -1,32 +1,12 @@
 #!/usr/bin/env bash
-# Ubuntu/Debian desktop quirks: apt + snap tier composition, batcat/fdfind
-# symlinks, pyenv build deps. The apt install/Tailscale/Docker hooks and apt
-# bootstrap are shared defaults in lib/core.sh.
+# Ubuntu/Debian desktop quirks: pyenv build deps. The apt bootstrap, install
+# tiers, snap, symlinks, and Tailscale/Docker hooks are all shared defaults in
+# lib/core.sh.
 
 platform_bootstrap() {
   apt_bootstrap
 }
 
-platform_install_tier() {
-  case "$1" in
-    high)
-      printf '==> Installing high-priority apt packages...\n'
-      apt_install_tier high
-      ;;
-    medium)
-      printf '==> Installing medium-priority apt packages...\n'
-      apt_install_tier medium
-      printf '\n==> Installing snap packages...\n'
-      snap_install_tier medium
-      setup_bat_fd_symlinks
-      ;;
-    low)
-      apt_install_tier low
-      snap_install_tier low
-      ;;
-  esac
-}
-
 platform_pyenv_build_deps() {
   sudo apt install -y libssl-dev libffi-dev libncurses-dev libreadline-dev \
     libbz2-dev libsqlite3-dev liblzma-dev zlib1g-dev tk-dev