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>

app/build.gradle.kts

@@ -61,6 +61,21 @@ android {
             applicationIdSuffix = ".debug"
             isMinifyEnabled = false
         }
+        // A locally-installable twin of `release`: same R8 shrinking + obfuscation
+        // and resource shrinking, but debug-signed and given its own applicationId
+        // suffix so it installs alongside both the production app (signed with the
+        // real key) and the debug build. Used to smoke-test a release candidate on
+        // a real device before tagging — R8-only breakage and first-run/permission
+        // states don't surface in the unminified debug build, nor on a device that
+        // already holds the permission. Never published. See docs/RELEASING.md.
+        create("releaseTest") {
+            initWith(getByName("release"))
+            applicationIdSuffix = ".releasetest"
+            signingConfig = signingConfigs.getByName("debug")
+            isMinifyEnabled = true
+            isShrinkResources = true
+            matchingFallbacks += "release"
+        }
     }
 
     compileOptions {