Build the APK, out of ClojureDart and Flutter

The shared half compiles under a second compiler, which was the thing worth
proving. `clojure -M:cljd compile` turns common/frq/{clock,store,io}.cljc and
flutter/src/frq/{main,io/dart}.cljd into 698 lines of Dart, and Flutter's
Gradle build packs it into an APK for all three ABIs. It installs and runs:
the clock and the saved session on screen, read through the same namespaces
`just tui` reads them through.

Two things had to give first.

frq.clock was not as portable as moving it to common/ implied. It reached for
Math/floorDiv and Math/floorMod, which are Java and have no equivalent under
ClojureDart — so the seam that exists for the filesystem was needed one scale
down, and floor-div is now eight lines of arithmetic. Checked against
java.time.LocalDate for every day from 1901 to 2052 and against Math/floorDiv
across the range, both zero mismatches. This is the shape of the rest of the
port: what breaks is never the language, it is the host underneath it.

And frq.main was written from the docs rather than from the compiler, so it
had cljd.flutter's nesting wrong — slot names are forms in a chain, not
arguments inside the widget, and MaterialApp takes .home where Center takes
.child. It also faked the await with a :watch over a Future; main is ^:async
now and awaits path_provider before installing the host, which is what the
docstring claimed all along.

The build is impure and stays that way. Gradle fetches its own dependencies
and installs build-tools and a platform into ANDROID_HOME, so it can neither
run in a sandbox nor write to the store; nix supplies the toolchain and the
recipe copies the SDK somewhere writable. No ndkVersion, because setting it
makes Gradle fetch an NDK for native code that does not exist here. ios/,
macos/, windows/ and web/ are deleted — android/ and linux/ are the targets.

Signed with ~/.android/debug.keystore, like every debug build. The jolt APK's
key was generated inside its own derivation and never written down, so the
first install over it needed an uninstall — Android will not update a package
across a signature change. Release signing is still the template's TODO.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>