ci(release): derive canary version from git tags (single source of truth)

Every release workflow hardcoded a canary base version (0.5.0 in
Apple/Android/rpm/flatpak/deb, 0.3 in windows-msix/windows-host/decky) that
had to be hand-bumped on each stable release and wasn't. With stable at
v0.6.0, every canary was a version *behind* stable — e.g. the Apple canary
showed up on TestFlight as 0.5.0 while 0.6.0 was already published.

Add scripts/ci/pf-version.{sh,ps1} (bash + pwsh twin) as the single source of
truth: stable = the vX.Y.Z tag; canary = latest stable tag with minor+1,
patch 0 (v0.6.0 -> 0.7.0), so canary is always exactly one minor ahead of the
newest release with zero maintenance. Falls back to the workspace Cargo.toml
version when no tag is fetchable. All workflows now eval/call it and format
their own channel suffix off $PF_BASE; only the canary branch changed, stable
branches and per-channel suffixes are untouched. channels.md drops the old
manual "bump the canary base" release step.

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

.gitea/workflows/release.yml

@@ -99,13 +99,14 @@ jobs:
 
       - name: Version from tag
         run: |
+          eval "$(bash scripts/ci/pf-version.sh)"   # -> PF_BASE, PF_CHANNEL, PF_STABLE_TAG (single source of truth)
           case "$GITHUB_REF" in
             refs/tags/v*) V="${GITHUB_REF_NAME#v}"; V="${V%%-*}" ;;  # App Store marketing version is numeric X.Y.Z (drop -rc)
-            *)            V="0.5.0" ;;                                # canary marketing version; the build number disambiguates
+            *)            V="$PF_BASE" ;;                             # canary marketing version = one minor ahead of the latest stable tag; the build number disambiguates
           esac
           echo "VERSION=$V" >> "$GITHUB_ENV"
           echo "BUILD_NUM=$GITHUB_RUN_NUMBER" >> "$GITHUB_ENV"
-          echo "version $V build $GITHUB_RUN_NUMBER"
+          echo "version $V build $GITHUB_RUN_NUMBER (channel $PF_CHANNEL, latest stable ${PF_STABLE_TAG})"
 
       - name: Rust toolchain (mac + iOS + tvOS slices)
         run: |