build,docs: add on-device release verification gate

Adds a mandatory pre-tag step to the release process: build the R8-shrunk
release candidate and smoke-test it on a real device, including a first-run /
permission-not-granted state. The v2.7.0 launch crash (calendar observer
registered before the permission gate) reached users because it only manifests
in the minified release build on a device without the permission already
granted — the debug build and an already-permissioned phone both hid it.

- New `releaseTest` build type: same R8 shrinking + obfuscation as `release`,
  but debug-signed with a `.releasetest` applicationId suffix so it installs
  alongside the production and debug apps. Never published; CI only ever builds
  the real `release` variant from the tag.
- scripts/verify-release.sh: builds + installs `releaseTest` and resets it to a
  first-run state, with an on-device checklist.
- docs/RELEASING.md: formalize the release/vX.Y.Z branch flow and the on-device
  verification gate before tagging.

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

scripts/verify-release.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+#
+# Build the release-candidate APK and install it on a connected device for the
+# mandatory pre-tag on-device check (see docs/RELEASING.md).
+#
+# It builds the `releaseTest` variant: the same R8 shrinking + obfuscation and
+# resource shrinking as the published `release` build, but debug-signed and
+# with a `.releasetest` applicationId suffix so it installs alongside the
+# production and debug apps. This is what surfaces release-only breakage (R8
+# stripping) and first-run states (permission not yet granted) that the
+# unminified debug build — or a device that already holds the permission —
+# silently hides.
+#
+# Usage: scripts/verify-release.sh
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+PKG="de.jeanlucmakiola.calendula.releasetest"
+APK="app/build/outputs/apk/releaseTest/app-releaseTest.apk"
+
+echo "==> Building release-candidate APK (releaseTest, R8 minified)…"
+./gradlew :app:assembleReleaseTest
+
+echo "==> Installing $PKG …"
+adb install -r "$APK"
+
+echo "==> Resetting to a first-run state (revoking calendar permission)…"
+# The release build crashed at launch precisely because this state was never
+# tested. Force it so the permission gate / onboarding is exercised every time.
+adb shell pm revoke "$PKG" android.permission.READ_CALENDAR  2>/dev/null || true
+adb shell pm revoke "$PKG" android.permission.WRITE_CALENDAR 2>/dev/null || true
+
+echo
+echo "Installed and reset. Now verify ON THE DEVICE before tagging:"
+echo "  1. Launch from a clean state — the permission screen must appear (no crash)."
+echo "  2. Grant calendar access — the calendar must load with events."
+echo "  3. Add both home-screen widgets and confirm they render (not a spinner)."
+echo "  4. Exercise the release's headline changes end to end."
+echo
+echo "Watch for crashes with:  adb logcat -b crash"
+echo "Only tag the release once all of the above pass on a real device."