Professionalize the C++ exercise repository

[doctorlai-msrc]

Summary

Professionalize the C++ exercise repository with a consistent local/CI verification interface, sanitizer-aware builds, aggregate coverage gates, secure pull request coverage comments, and complete contributor-facing repository documentation.

Changes

• Add top-level build, test, format, lint, coverage, and check Make targets.
• Run clean sanitizer builds so object files cannot be reused across incompatible compiler flags.
• Use fail-fast AddressSanitizer and UndefinedBehaviorSanitizer checks locally; run ThreadSanitizer on supported native Linux environments.
• Detect WSL2 and skip the unsupported GCC ThreadSanitizer runtime there by default while retaining the full ASan/TSan/UBSan matrix in GitHub Actions.
• Propagate shared linker flags through every example Makefile so sanitizer and coverage runtimes are linked consistently.
• Generate text, HTML, XML, and JSON coverage reports with minimum coverage gates of 80% lines, 80% functions, and 70% branches.
• Upload coverage artifacts and create or update a coverage summary comment on pull requests.
• Keep the privileged coverage-comment workflow isolated from PR code, validate all artifact metrics, resolve fork PRs through the GitHub API, reject stale commits, serialize updates, and pin actions to immutable commit SHAs.
• Replace the bc-based Pi checks with strict numeric awk validation.
• Fix escaped newline output in the smart pointer and Sudoku examples.
• Add .gitattributes, generated-artifact ignores, Dependabot configuration, issue/PR templates, and contributor, security, support, privacy, and changelog documentation.
• Refresh the README with the complete example inventory, supported commands, dependencies, CI behavior, coverage policy, badges, and WSL2 sanitizer notes.
• Update repository sponsorship options in .github/FUNDING.yml.

Coverage

Category	Result	Required
Lines	91.4% (755 / 826)	80%
Functions	98.9% (87 / 88)	80%
Branches	71.3% (469 / 658)	70%

Validation

• make check on Ubuntu 24.04 under WSL2: passed with ASan and fail-fast UBSan; TSan was skipped as documented because the GCC runtime cannot reliably reserve its shadow-memory layout under WSL2.
• make coverage: passed all aggregate thresholds and generated text, HTML, XML, and JSON reports.
• Injected recursive clean, formatting, and compiler failures propagate as nonzero exits.
• GitHub workflow and Dependabot YAML parsing: passed.
• Embedded actions/github-script JavaScript syntax check: passed.
• Shell syntax checks for clang-check.sh and all tests.sh files: passed.
• README local-link and example-inventory checks: passed.
• Editor diagnostics: no errors.
• git diff HEAD --check: passed.

Notes

• GitHub Actions explicitly runs ASan, TSan, and UBSan on native Ubuntu runners; native TSan could not be executed locally from this WSL2 workspace.
• The workflow_run coverage-comment path, including fork PR association and concurrency cancellation, requires an actual pull request to verify end to end on GitHub.

[Copilot]

Pull request overview

This PR professionalizes the C++ examples repository by standardizing local developer commands, hardening/expanding CI (sanitizers + coverage + PR coverage comments), and adding contributor-facing repository documentation and housekeeping files.

Changes:

• Introduces a consistent top-level make interface (build/test/format/lint/coverage/check), sanitizer-aware clean builds, and aggregate coverage reporting/gates.
• Expands GitHub Actions into dedicated sanitizer and coverage jobs, plus a separate workflow_run-based coverage comment workflow with pinned action SHAs.
• Updates examples and docs: propagates shared linker flags in example Makefiles, fixes escaped newline output in examples, replaces bc with strict awk validation in Pi tests, and adds repo policy docs/templates.

Reviewed changes

Copilot reviewed 36 out of 37 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Makefile	Adds top-level build/test/lint/coverage/check targets, WSL-aware sanitizer matrix defaults, and aggregate coverage generation via gcovr.
common.mk	Adds COVERAGE switch for --coverage flags and makes UBSan fail-fast with -fno-sanitize-recover=undefined.
.github/workflows/ci.yml	Splits CI into ASan/UBSan, dedicated TSan (with instrumented oneTBB), and coverage jobs; pins actions by SHA.
.github/workflows/coverage-comment.yml	Adds isolated workflow_run workflow to post/update PR coverage summary comments from artifacts.
.gitignore	Ignores additional build/coverage artifacts (deps, gcov outputs, binaries, coverage directory).
.gitattributes	Enforces LF EOL normalization across common source/config file types.
README.md	Expands documentation for prerequisites, commands, sanitizers (incl. WSL2 notes), CI behavior, and repo meta/badges.
CONTRIBUTING.md	Adds contributor workflow, prerequisites, and required local validation commands.
SECURITY.md	Adds a security policy and private reporting guidance.
SUPPORT.md	Adds support channels and guidance for issues/discussions/security reports.
PRIVACY.md	Documents repository privacy expectations and cautions about sharing sensitive data in public channels.
CHANGELOG.md	Introduces a changelog entry describing the repo “professionalization” changes.
.github/PULL_REQUEST_TEMPLATE.md	Adds PR template with validation and checklist items aligned with new make targets.
.github/ISSUE_TEMPLATE/bug_report.md	Updates bug report template to request repro commands, sanitizer config, and environment details.
.github/FUNDING.yml	Updates sponsorship configuration links.
.github/dependabot.yml	Adds Dependabot configuration for GitHub Actions updates.
virtual-interface/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS) (sanitizers/coverage runtime linkage).
views-zip-enumerate/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
unique-ptr-basics/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
thread-safe-queue/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
sudoku-solver/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
sudoku-solver/main.cpp	Fixes escaped newline output to print real line breaks in usage/errors/results.
smart-ptr/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
smart-ptr/main.cpp	Fixes escaped newline output to print real line breaks in test output.
rot47/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
ranges-find-in-vector-string/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
prefix-sum/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
pi/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
pi/tests.sh	Replaces bc comparisons with strict numeric validation and range checks via awk; uses safer bash flags.
pi-monte-carlo/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
pi-monte-carlo/tests.sh	Replaces bc comparisons with strict numeric validation and range checks via awk; uses safer bash flags.
parallel-integral-pi/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
oop-with-exception/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
multithread-sum/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
map-with-unknown-key/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
integer-factorization/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).
fold-left-fold-right/Makefile	Ensures link rules honor shared $(LDFLAGS)/$(LDLIBS).

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