test(tiers): T504 Host tier guard, workflow tier comments, CONTRIBUTING tier table (gh-#777)

[genwave-radio]

PR-10 of gh-#777 · STORY-439 · T504

🧪 What changes

• Host tier guard (SPEC F177.3): TierTraitConventionGuard.FindViolations walks the assembly by reflection and reports any test class that consumes KokoroFixture or a stack fixture (today: RequestsArtworkFlywheelFixture) without Category=Integration at class level or on every fact. Detection covers IClassFixture<>, ICollectionFixture<>, [Collection(name)] resolved to its definition, and constructor parameters. HostTierTraitConventionGuard runs it on every PR. The MediaLibrary guard is untouched and its hash is pinned.
• One offender found: Story230's flywheel scenario needs the live compose stack, so it now carries the trait and runs nightly instead of on PRs. Its runtime-skip idiom is unchanged.
• Workflow comments (F177.2): ci.yml and release.yml no longer say integration tests run "on-demand on a dev box"; they state the three tiers. No run: line or filter changed (F177.4).
• CONTRIBUTING gains a "Test tiers" table naming the three filters and where each runs.
• Story439 pins: 10 facts, no longer skipped.

✅ Proof

• Full solution green (builder and reviewer runs): Host 3064 passed / 57 skipped, 0 failed across 9 projects.
• dotnet test tests/GenWave.Host.Tests --filter "Category=Integration" --list-tests lists the two flywheel facts, so they moved to tier 2.
• actionlint clean on both workflows.

🔍 Review

One round, PASS-WITH-NOTES (a CONTRIBUTING row implied release.yml runs per PR, a stale "red at plan time" header, a missing note that inherited facts are not checked). Fixed as doc edits. The root guard class is named HostTierTraitConventionGuard because a root TierTraitConventionGuard would shadow the Support type inside the Specs namespace.