Cache Rust build artifacts in CI

[kwsantiago]

Problem

Every CI run rebuilds the Rust native libraries from scratch. On a clean runner this is a measurable chunk of each PR's wall time.

Proposal

ci.yml already uses Swatinem/rust-cache scoped to keep/keep-mobile, but does not cache the compiled .so outputs or the generated bindings, so build-rust.sh always redoes the cargo-ndk build and binding generation.

Extend the existing setup:

1. Broaden Swatinem/rust-cache workspaces to include the whole keep workspace, not just keep-mobile, so transitive crate builds are cached.
2. Add an actions/cache step for the build outputs:
   • Paths: app/src/main/jniLibs/, app/src/main/kotlin/io/privkey/keep/uniffi/
   • Key: hash of keep commit SHA + Cargo.lock + NDK version + TARGETS env
3. On cache hit, skip the ./build-rust.sh step entirely.

Acceptance

• Cache hit on back-to-back PRs skips recompilation and binding generation
• Cache invalidates correctly when keep SHA, Cargo.lock, or NDK version change
• Applies to both ci.yml and release.yml

References

• Swatinem/rust-cache: already wired in ci.yml, extend rather than replace
• actions/cache: for the compiled-output layer
• ianthetechie/uniffi-starter: CI caches cargo and generated bindings in a similar sibling-workspace layout

Follow-up from #221.