android: the binding generator gets its own crate, free of the app's deps

Third Android run got further than either before it — all four ABIs
cross-compiled, vendored OpenSSL and all — then the generator died:

  error: failed to run custom build command for `openssl-sys v0.9.117`

That is the HOST build. The generator was a [[bin]] inside thoughtsync-ffi, so
building it compiled that crate and therefore the core, reqwest, native-tls and
openssl-sys for linux. The vendored-OpenSSL block is scoped to
`cfg(target_os = "android")`, so the host build went looking for a system
OpenSSL that ci-rust-android has no reason to carry.

Adding libssl-dev to the image would have fixed it and been wrong: a code
generator has no business linking the app's TLS stack to emit Kotlin. Splitting
it into thoughtsync-uniffi-bindgen, whose only dependency is uniffi, removes the
entire chain. Verified from the dependency graph rather than from a build that
happened to succeed — `cargo tree -p thoughtsync-uniffi-bindgen` contains none of
openssl-sys, native-tls, reqwest, thoughtsync-core or rusqlite.

It stays a WORKSPACE MEMBER on purpose. Sharing one lockfile is what keeps uniffi
here and uniffi linked into the .so at one version; they are two halves of one
ABI, and a separate lockfile is precisely how they would drift apart. The cost is
that the desktop lane now compiles ~15 generator crates it never runs — cheap
next to Tauri, and better than leaving the crate unlinted.

Drops the `bindgen` feature and required-features bin from thoughtsync-ffi, which
existed only to keep those crates off the desktop lane and now have nothing to
gate.

Local fmt + clippy + test all green before pushing (107 tests).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -4213,6 +4213,13 @@ dependencies = [
  "uniffi",
 ]
 
+[[package]]
+name = "thoughtsync-uniffi-bindgen"
+version = "0.1.0"
+dependencies = [
+ "uniffi",
+]
+
 [[package]]
 name = "time"
 version = "0.3.55"