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>

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "floret-kit"]
+	path = floret-kit
+	url = https://gitea.jeanlucmakiola.de/makiolaj/floret-kit.git