bakery: fix correctness, reliability, and security issues from audit

Track switches now always take effect on `update --all` instead of
silently no-op'ing or permanently refusing on strict semver comparison.
`remove` no longer aborts cleanup on the first failed binary removal,
orphaning the systemd unit. State reads/writes are now lock-protected
and go through fsync'd atomic writes (also fixes a temp-path collision
in binary installs). The index loader falls back to a stale-but-signed
cache instead of hard-failing offline. systemd units now re-fetch on
every update instead of freezing after first install. `doctor` now
flags missing recorded binaries.

Security hardening: path-traversal guard on all index-controlled
filenames, archive extraction now rejects symlink/traversal entries
before tar touches disk, archive temp files use secure unique paths,
post_install hooks are gated behind --no-hooks/confirmation, response
buffering is capped, empty-checksum downloads get a clear error, and
both stable-track CI workflows now hard-fail on a missing signing key
(matching the existing dev/rc guard) instead of silently publishing an
index next to a stale signature. gen-index.sh now publishes the index
and its signature atomically.

Also: bakery install on an already-installed package no longer
silently reinstalls/downgrades, cmd_update exits non-zero for unknown
packages, and the unused toml dependency is removed.

.forgejo/workflows/release-bread-theme.yml

@@ -57,7 +57,13 @@ jobs:
       - name: regenerate index.json
         env:
           MINISIGN_SEC_KEY: ${{ secrets.BAKERY_MINISIGN_SEC_KEY_PATH }}
-        run: cd src && bash scripts/gen-index.sh
+        run: |
+          set -euo pipefail
+          if [ -z "${MINISIGN_SEC_KEY:-}" ]; then
+            echo "::error::BAKERY_MINISIGN_SEC_KEY_PATH secret not set — refusing to regenerate stable index.json unsigned (would leave a stale signature mismatched against fresh content and break bakery for everyone on the stable track)"
+            exit 1
+          fi
+          cd src && bash scripts/gen-index.sh
 
       - name: upload to GitHub Release
         env: