Cache NuGet packages in CI

[Trosper3]

Summary

• .github/workflows/main.yml used actions/setup-dotnet@v6 with no NuGet caching, so every PR run did a full cold restore.
• Adds cache: true with an explicit cache-dependency-path pointing at the repo's two .csproj files (API.csproj, redux-tests/redux-tests.csproj) — there's no packages.lock.json in this repo, so the default lock-file glob wouldn't pick up a cache key; pointing at the project files directly means the cache still invalidates correctly when dependencies change.

Closes #460

Test plan

• YAML validated (yaml.safe_load)
• dotnet build Redux.slnx --configuration Release on the rebased branch — clean, 0 warnings/errors
• Actual cache hit/restore-time improvement will show up on this PR's own CI run and subsequent ones

[github-actions]

Redux Build System — CI Report

dotnet · commit b4a6e05

Operation	Status	Summary	Time
audit	✅	no known vulnerabilities	4.5s
format-check	✅	all files formatted	23.1s
lint	✅	0 issues	36.9s
typecheck	⏭️	covered by lint (dotnet build)	—
unit-test	❌	898 passed · coverage 70% (min 80%)	45.3s
build	✅	built local/redux:ci · 234MB	40.7s
integration-test	⏭️	no [integration] command	—
push	⏭️	not implemented	—

Overall: ❌ 4 passed · 1 failed · 3 skipped