Native core bindings for Rocksky.

JVM Panama (java.lang.foreign) bindings to the shared Rust core's C ABI
(crates/rocksky-uniffi). AT Protocol PDS writes (scrobble fan-out, like,
follow, shout), AppView reads, and the identity hashes shared across every
Rocksky SDK. This is the write + dedup side; `rocksky.client` is the read/HTTP
side. Requires JDK 22+ (run with --enable-native-access=ALL-UNNAMED).

Resolves the native rocksky-uniffi library, downloading a prebuilt from the
GitHub release on first use when it isn't already present locally.

Order of preference:
  1. $ROCKSKY_NATIVE_LIB, if set.
  2. librocksky_uniffi.<ext> on the classpath (resources/) — a local
     ./build-core.sh dev build.
  3. A checksum-verified copy in the user cache, downloaded on first load (the
     published-jar path: the native lib is not bundled in the jar).

The jar ships resources/rocksky/manifest.json (repo, release tag, one sha256
per target triple) — filled from the release artifacts by
sdk/scripts/gen-uniffi-manifest.sh.