Add UndefinedBehaviorSanitizer build/test option

[tameware]

Summary

• Add standalone --config=ubsan (not combined with ASan) with macOS Xcode compiler-rt rpath, matching the TSan pattern
• Wire UBSan into Linux CI (ubsan job on //library/tests/...) and macOS sanitizers job
• Fix UBSan-found undefined behavior: rank_[-1] in lookup-table init, first[-4] on empty deals in solve_board, and fullseq_[-1] in get_top_number for empty suits

Test plan

• bazelisk test --config=ubsan //library/tests/... (41/41 pass locally on macOS)
• Linux CI ubsan job green
• macOS CI sanitizers job includes UBSan step and stays green
• Spot-check docs: docs/BUILD_SYSTEM.md UBSan section matches .bazelrc

Made with Cursor

[Copilot]

Pull request overview

Adds a standalone UndefinedBehaviorSanitizer (UBSan) Bazel config and wires it into CI, then fixes several UBSan-reported undefined behaviors in core lookup-table initialization, solver initialization, and heuristic sorting edge cases.

Changes:

• Add --config=ubsan in .bazelrc, document it, and expose it in Copilot instructions.
• Run UBSan in GitHub Actions (Linux: new ubsan job; macOS: add UBSan step in sanitizers job).
• Fix undefined behavior in:
  • lookup-table group gap computation (rank_[-1] case),
  • solver look-ahead first[] indexing on invalid/degenerate deals,
  • get_top_number() for empty suits (fullseq_[-1] case), plus targeted tests.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
library/tests/utility/lookup_tables_test.cpp	Adds regression coverage for empty/single-card suit group initialization invariants.
library/tests/heuristic_sorting/targeted_unit_tests.cpp	Tightens edge-case expectation for empty-suit get_top_number() behavior.
library/src/solver_if.cpp	Avoids invalid first[] indexing before last-trick / validation handling.
library/src/lookup_tables/lookup_tables.cpp	Prevents rank_[g-1] access when opening the first group from an empty suit.
library/src/heuristic_sorting/heuristic_sorting.cpp	Guards empty-suit get_top_number() to avoid fullseq_[-1] access.
docs/BUILD_SYSTEM.md	Documents UBSan usage and macOS runtime coupling (matching TSAN pattern).
.github/workflows/ci_macos.yml	Adds UBSan fetch + test step to the macOS sanitizers workflow.
.github/workflows/ci_linux.yml	Adds a dedicated Linux UBSan CI job running //library/tests/....
.github/copilot-instructions.md	Adds the UBSan Bazel config command to the contributor guidance.
.bazelrc	Introduces --config=ubsan and macOS rpath wiring for UBSan runtime loading.