Add single-threaded WASM build for z3-solver without SharedArrayBuffer

- Add makeCCSinglethreadWrapper() in make-cc-wrapper.ts that generates
  synchronous async wrappers using EM_ASM instead of threads +
  MAIN_THREAD_ASYNC_EM_ASM. Refactor shared error-handler code into a
  SHARED_ERROR_HANDLERS constant used by both variants.

- Add build-wasm-singlethread.ts: new build script that compiles a
  separate libz3.a (in build-singlethread/) and z3-built-singlethread.js
  without -pthread / USE_PTHREADS=1. Uses -DSINGLE_THREAD -DPOLLING_TIMER
  and ALLOW_MEMORY_GROWTH instead of a fixed 2 GB SharedArrayBuffer.

- Add src/node-singlethread.ts and src/browser-singlethread.ts entry
  points. Browser variant reads initZ3SingleThread from global scope
  (load z3-built-singlethread.js via importScripts in a Web Worker).

- Add tests for both new entry points (node-singlethread.test.ts,
  browser-singlethread.test.ts).

- Update package.json: add build:wasm:singlethread script; add exports
  map so users can do import { init } from 'z3-solver/singlethread'.

- Update PUBLISHED_README.md: document the single-threaded variant,
  explain when to use it (no COOP/COEP headers), and provide a Web
  Worker usage example.

- Update wasm.yml CI: add check-singlethread parallel job.
- Update wasm-release.yml: build single-threaded artifact before publish.

- Guard scoped_timer.cpp with #if defined(SINGLE_THREAD) &&
  !defined(__EMSCRIPTEN_PTHREADS__) so that thread-based timer creation
  is skipped in single-threaded Emscripten builds (POLLING_TIMER covers
  the primary cancel_eh<reslimit> use-case).

.github/workflows/wasm-release.yml

@@ -48,7 +48,7 @@ jobs:
       - name: Build TypeScript
         run: npm run build:ts
 
-      - name: Build wasm
+      - name: Build wasm (threaded)
         run: |
           emsdk install ${EM_VERSION}
           emsdk activate ${EM_VERSION}
@@ -57,6 +57,11 @@ jobs:
           which clang++
           npm run build:wasm
 
+      - name: Build wasm (single-threaded)
+        run: |
+          source $(dirname $(which emsdk))/emsdk_env.sh
+          npm run build:wasm:singlethread
+
       - name: Test
         run: npm test