Code Quality Fixes: Dead Code, Access Modifiers, and Cache Leak

Internal code quality fixes: removes production rendering heuristics tied to test fixtures, eliminates dead scaffolding code, corrects access modifiers, and closes a [ThreadStatic] cache leak.

🐛 Bug fixes

• Production snapshot heuristics removed (ReportRenderer.cs): Deleted IsKnownAfterApplyCompatibilityScenario and IsEphemeralOpenCompatibilityScenario helper methods that fingerprinted test-fixture resource counts to change table formatting in production builds. The heuristics were originally added as compatibility shims and had been dead weight since the scenarios they handled were properly covered.

• ShouldUseEphemeralOpenFormatting stub removed (DefaultResourceRenderer.cs): Deleted a permanently-dead formatting scaffolding — the method always returned false, making every branch it guarded unreachable. The ephemeral-open formatting path, the useEphemeralOpenFormatting variable, and the corresponding IsEphemeralOpenScenario property on IScenarioRenderContext/ScenarioRenderContext are all gone. ResolveScenarioFormatting return tuple narrowed from 3 to 2 elements.

• MarkdownHelpers visibility corrected (14 files): All 14 partial-class declarations of MarkdownHelpers were incorrectly public. Changed to internal to match the project specification. tfplan2md is a CLI tool, not a library; the test project already had InternalsVisibleTo access, so no tests are affected.

• ClearLineDiffCache() cache leak fixed (MarkdownRenderer.cs): Both Render() overloads now call MarkdownHelpers.ClearLineDiffCache() in a try/finally block. The [ThreadStatic] LCS (Longest Common Subsequence) cache was documented to require cleanup after each render pass, but the call was inadvertently lost when the Scriban renderer was removed. Under sustained load this caused unbounded memory growth per render thread.

• ApplyViewModel() dead stubs removed (IResourceViewModelFactory and implementations): Added a default no-op body to the IResourceViewModelFactory.ApplyViewModel() interface method, eliminating the need for pure-stub overrides. Deleted 4 stub implementations (NetworkSecurityGroupFactory, RoleAssignmentFactory, VariableGroupFactory, BuildDefinitionFactory) that contributed nothing. Real implementations (those building SummaryHtml/ChangedAttributesSummary) are preserved.

🔗 Commits

• f9f7682 fix: remove snapshot-compatibility heuristics and dead code from rendering pipeline
• f09fb4d fix: remove 5 code review findings (dead code, visibility, cache leak)