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>

docs/fdroid-official/de.jeanlucmakiola.calendula.yml

@@ -43,11 +43,21 @@ Builds:
     versionCode: 20705
     commit: v2.7.5
     subdir: app
+    submodules: true
     gradle:
       - yes
     # No NDK / no flavors. The release buildType applies a signingConfig only
     # when key.properties exists; on the buildserver it does not, so this
     # produces the unsigned APK F-Droid compares against our binary.
+    #
+    # submodules: true — REQUIRED from v2.11.0 onward, where the build pulls
+    # shared code from the `floret-kit` git submodule via a Gradle composite
+    # build (`includeBuild("floret-kit")`). Without it F-Droid checks out an
+    # empty floret-kit/ and the from-source build fails, stalling publishing.
+    # Inert for v2.7.5 (no submodule yet); kept here so AutoUpdateMode copies it
+    # onto every auto-generated future build entry. The kit is plain-Kotlin and
+    # carries no foojay toolchain resolver, so it clears the same reproducibility
+    # bar (enforced by scripts/check_reproducible_release.sh).
 
 # SHA-256 of our app signing certificate (public; embedded in every published
 # APK). Locks F-Droid to publish only binaries signed with our key.