stillwater-sc / universal / 21384226656

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V3.93: UBSan fixes, image demo, and coverage workflow (#499)

* Incrementing SEMVER to v3.93.1

* Fix UBSan errors: signed overflow and shift-too-large

1. integers.hpp ipow(): Use unsigned arithmetic internally to avoid
   signed integer overflow UB when computing large powers like 100^100

2. posit_32_2.hpp round_mul(): Add early return when scale > 30 to
   avoid shifting 32-bit int by 60+ bits (UB)

3. posit_32_2.hpp adjustAndRound(): Same fix - early return when
   reglen > 30 to avoid shift UB

All three issues were caught by UBSan in CI sanitizer workflow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add ULP visual demo for image precision analysis

- Add stb_image.h and stb_image_write.h to include/sw/image/
  (public domain, header-only image I/O libraries)
- Add ulp_visual_demo application that strips LSBs from RGB pixels
  and measures visual quality impact via PSNR metrics
- Demonstrates that lower 2-3 bits of 8-bit pixels often contain
  imperceptible detail, validating mixed-precision approaches

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Fix ulp_visual_demo to auto-create output directory

Added std::filesystem support to create the output directory if it
doesn't exist, preventing "FAILED TO WRITE" errors.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add code coverage workflow for Coveralls integration

Creates GitHub Actions workflow that:
- Builds with GCC coverage flags (--coverage)
- Runs CI test suite
- Generates lcov coverage report
- Uploads to Coveralls.io

This makes the existing Coveralls badge in README.md operational.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* update .gitignore to ignore result directories

* Fix coverage workflow lcov errors with modern GCC

Add --ignore-errors flags for:
- gcov: Unexecuted block warnings
- negative: Unexpected negative count errors (GCC 13+)
- unused: Unused coverage data after filtering

Co-Authored-By: Claude Opus 4.5 <norepl... (continued)

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