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>

.forgejo/workflows/android.yml

@@ -1,97 +0,0 @@
-# Android (Tauri v2 mobile) — SEPARATE from desktop.yml because it is a different
-# surface with a different toolchain, not a different bundle of the same one. The
-# Rust core is cross-compiled with the NDK and then packaged by the Gradle project
-# in desktop/src-tauri/gen/android.
-#
-# Toolchain comes from the ci-tauri-android image (Rust + the Android ABIs +
-# SDK/NDK + JDK 17 + tauri-cli); runs-on is only a scheduling label (Label Model B).
-#
-# Android, desktop and web are peer surfaces on one quality bar, so this triggers on
-# the same shared frontend that desktop.yml does — the frontend is compiled INTO the
-# app by generate_context!, so a frontend commit that skipped this lane would ship a
-# stale phone build.
-name: Android (Tauri)
-
-on:
-  push:
-    branches: [dev, main]
-    tags: ["v*"]
-    paths:
-      - "desktop/**"
-      - "frontend/**"
-      - ".forgejo/workflows/android.yml"
-  workflow_dispatch:
-
-concurrency:
-  group: android-${{ github.ref }}
-  cancel-in-progress: ${{ !startsWith(github.ref, 'refs/tags/') }}
-
-permissions:
-  contents: read
-
-jobs:
-  build:
-    name: Android APK (debug)
-    runs-on: python-ci
-    container:
-      image: git.fabledsword.com/bvandeusen/ci-tauri-android:1.97
-    steps:
-      - uses: actions/checkout@v6
-
-      # Same reason as the desktop lanes: generate_context! embeds the built
-      # frontend at compile time, so it must exist before cargo runs.
-      - name: Build the shared frontend
-        run: npm ci && npm run build
-        working-directory: frontend
-
-      # The lockfile gate, matching the desktop lanes (issue 2102). Fetching for the
-      # Android target also pre-warms exactly the crates the build will want.
-      - name: Verify the lockfile and fetch dependencies
-        run: cargo fetch --locked --target aarch64-linux-android
-        working-directory: desktop/src-tauri
-
-      # DEBUG, and arm64 only, deliberately.
-      #
-      # Debug because a release APK has to be signed, and the release keystore must
-      # be generated by the operator and never pass through CI logs or an agent
-      # session (the constraint recorded on task 2136 for the updater key applies
-      # identically here). Gradle's throwaway debug keystore needs nothing from
-      # anyone, so this lane can prove the app COMPILES and PACKAGES today and grow
-      # a signed release job when a keystore exists.
-      #
-      # arm64 + x86_64. arm64 is every real device; x86_64 is what a desktop
-      # emulator runs, and an arm64-only APK installs there and then dies unable to
-      # load its native library — so a build nobody can try on an emulator is a
-      # build nobody checks. armv7 and i686 are left out: 32-bit hardware we do not
-      # target, and the image carries all four if that changes.
-      - name: Tauri build (Android APK)
-        run: |
-          version="$(sh ../packaging/build-version.sh)"
-          echo "Building version $version"
-          cargo tauri android build \
-            --debug \
-            --target aarch64 \
-            --target x86_64 \
-            --config '{"build":{"beforeBuildCommand":""}}' \
-            --config "{\"version\":\"$version\"}"
-        working-directory: desktop/src-tauri
-
-      # Located rather than hardcoded: the output path depends on the target and
-      # build type, and a wrong literal here would fail as "no files found" long
-      # after the expensive step succeeded.
-      - name: Locate the APK
-        run: |
-          apk="$(find desktop/src-tauri/gen/android/app/build/outputs/apk -name '*.apk' -type f | head -1)"
-          [ -n "$apk" ] || { echo "ERROR: no APK produced" >&2; exit 1; }
-          echo "Built $apk ($(du -h "$apk" | cut -f1))"
-          echo "APK_PATH=$apk" >> "$GITHUB_ENV"
-
-      # Mirrored action, never actions/upload-artifact — see desktop.yml's Upload
-      # bundles step for the full reasoning. Pinned by SHA because the mirror
-      # auto-syncs.
-      - name: Upload the APK
-        uses: https://git.fabledsword.com/bvandeusen/upload-artifact@cb8afe72b42edc798abfb8fcb556cf660d894245
-        with:
-          name: thoughtsync-android-debug
-          path: ${{ env.APK_PATH }}
-          if-no-files-found: error