core: extract the store and sync engine into a shared crate (M12 step 1)

Android becomes a native Kotlin client over this same code (Scribe note 2730), so
the local store and sync engine stop being modules of the desktop app and become
`thoughtsync-core`, a crate with no UI framework in it at all.

This is a move, not a rewrite, and the measurement is why: every file in local/
and sync/ already carried ZERO Tauri references — 4,980 of 6,372 lines. The
coupling was 473 lines of command shim, which stays behind in the desktop crate
as src/commands/. Kept as git renames so history follows the files.

The desktop imports them under their old names (`use thoughtsync_core::{local,
sync}`) so every call site reads exactly as before. What moved is where they
live, not what they are.

Two things a workspace changes that are easy to miss, both caught before pushing:

[profile.release] now lives at the workspace ROOT. Cargo silently ignores
profiles declared by a non-root member — leaving it in the desktop crate would
have dropped lto/strip/opt-level from every release build with only a warning.

And a workspace shares ONE target dir, so the bundles moved from
desktop/src-tauri/target to target/. Thirteen references across publish-release,
debundle-graphics, verify.sh, package-prebuilt and the workflow now point there.
Pinning target-dir back would have been the smaller diff, but the Android lane
also produces Rust artifacts and they do not belong under desktop/.

Also retires the Tauri Android lane in the same push rather than leaving a path
that is being replaced: gen/android, android.yml and docs/android-dev.md are
gone, the mobile_entry_point attribute with them, and the lib drops to rlib —
staticlib/cdylib existed for Tauri mobile, and the .so Android loads will be
built from the core crate instead. Rule 22, no parallel path.

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

core/src/sync/mod.rs

@@ -0,0 +1,22 @@
+//! Talking to a ThoughtSync server — entirely opt-in.
+//!
+//! The app is local-first: `local` is the source of truth and everything works
+//! unlinked. Nothing in here runs until the user links a server.
+//!
+//! - `compat` — the version/capability handshake (M10.6): whether a given server can
+//!   be talked to at all. Pure decision logic, no I/O.
+//! - `client` — HTTP transport: the handshake call and device-token auth.
+//! - `state` — the persisted link record (server, token, change-feed cursor).
+//! - `engine` — one full cycle: push local changes, then pull the server's.
+//!
+//! The UI surface that drives this lives in whichever client is wrapping the crate,
+//! not here.
+
+pub mod blobs;
+pub mod client;
+pub mod compat;
+pub mod engine;
+pub mod pull;
+pub mod push;
+pub mod state;
+pub mod wire;