Add single-threaded PAGX Viewer builds

[fengdyli]

Summary

• Add single-threaded PAGX Viewer build scripts alongside the existing multi-threaded builds.
• Let Rollup select the wasm output flavor through ARCH and copy artifacts from the matching directory.
• Preserve release wasm symbol maps from the CMake build output.
• Document the new single-threaded build commands and output overwrite behavior.

Testing

• Not run in this session.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.59%. Comparing base (b54da6f) to head (0a38c04).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3435      +/-   ##
==========================================
+ Coverage   78.49%   81.59%   +3.09%     
==========================================
  Files         532      558      +26     
  Lines       41448    51406    +9958     
  Branches    12486    13987    +1501     
==========================================
+ Hits        32536    41944    +9408     
- Misses       6383     6598     +215     
- Partials     2529     2864     +335     

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