build: verify release inputs

Pin and verify what goes into a release: JitPack confined to the
SPAKE2 module via exclusiveContent, the vendored subtree pulled by
commit SHA instead of a mutable tag, a pinned Docker base digest with
a checksummed cmdline-tools download, and a verifyScrcpyServer task
wired into every assets merge. The expected server checksum can be
overridden with -PscrcpyServerSha256 when the jar is built from
source.

build-apk now requires apksigner and fails unless independent
signature verification succeeds. Split the test image into unit and
e2e targets so JVM-only test runs do not download an emulator. Ship
THIRD_PARTY_NOTICES and the LGPL text for SPAKE2 in the APK. Add
scripts/check as the host quality gate.

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