Draw core-time from floret-kit (TimeBridge) via submodule + composite build

Phase 1 of the shared-kit plan: Calendula becomes the second core-time consumer.
floret-kit is embedded as a git submodule and wired with includeBuild, so the
app depends on de.jeanlucmakiola.floret:core-time and Gradle substitutes the
local source module (built from source).

- Remove the app's TimeBridge (Instant <-> epoch millis) + its test; the helpers
  now live in core-time. Repoint the 7 call sites to de.jeanlucmakiola.floret.time.
- Calendula's richer time formatting (TimeFormat, 24h CompositionLocal) stays
  app-local by design.

Reproducible-build handling (Calendula publishes via official F-Droid repro):
- Pin the submodule to a foojay-free kit commit (the resolver would let a build
  fetch a JDK -> non-reproducible; the kit never used a toolchain block).
- Harden scripts/check_reproducible_release.sh to scan the included build's
  Gradle scripts for the toolchain-resolver invariant.
- Draft fdroiddata recipe: submodules: true (else F-Droid checks out an empty
  floret-kit/ and the from-source build fails).
- CI build checkouts fetch submodules recursively.

Clean composite build + unit tests + lintDebug + repro guard all green.

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

scripts/check_reproducible_release.sh

@@ -34,8 +34,15 @@ else
 fi
 
 # 2. The foojay toolchain resolver must not be present in any Gradle script.
+#    This includes the floret-kit submodule: it's an included build (composite
+#    build via `includeBuild`), so F-Droid evaluates its Gradle scripts too when
+#    building from source — the same offline-scanner bar applies to it.
 gradle_files=("$SETTINGS" "$APP")
 [ -f build.gradle.kts ] && gradle_files+=(build.gradle.kts)
+if [ -d floret-kit ]; then
+  while IFS= read -r f; do gradle_files+=("$f"); done \
+    < <(find floret-kit -name '*.gradle.kts' -not -path '*/build/*')
+fi
 if grep -qi 'foojay' "${gradle_files[@]}"; then
   echo "ERROR: foojay toolchain resolver found in: $(grep -li foojay "${gradle_files[@]}" | tr '\n' ' ')" >&2
   echo "       F-Droid's source scanner rejects org.gradle.toolchains.foojay-resolver" >&2