Make the node row label agree with its percentage

[erikdarlingdata]

Summary

• The plan viewer's node label printed both row counts with N0 but took the percentage from the unrounded values. When rounding moved a number, the label contradicted its own percentage. A Key Lookup that ran 117 times for 1 row read "1 of 1 (89%)".
• The new PlanRowAccuracy class applies the rule from the issue. It prints N0 first. If the whole percent of the printed numbers differs from the printed percentage, it adds the fewest decimals at which they agree, up to 4. It never prints a non-zero value as 0. That label now reads "1 of 1.128 (89%)".
• Labels whose rounded numbers already agree do not change.
• The class is a port of PerformanceMonitor's PlanRowAccuracy (Plan viewer: a screen-reader select on the + tab adds one sub-tab, and a row label under 1 agrees with its percentage (#4684) PerformanceMonitor#4688 and Plan viewer and analyzer: compare row estimates the way PerformanceStudio does (#4627 follow-up) PerformanceMonitor#4698), so both apps print the same string for the same plan.

Closes #611

Changes

• src/PlanViewer.Core/Services/PlanRowAccuracy.cs (new): FormatActualOfExpected returns the whole label. PrintActualOfExpected returns the two numbers and the percentage, for a surface that words the label differently.
• src/PlanViewer.App/Controls/PlanViewerControl.Rendering.cs and src/PlanViewer.Web/Pages/Index.razor: the App and Web node labels use FormatActualOfExpected. The label color does not change.
• src/PlanViewer.Core/Output/HtmlExporter.cs: the HTML export uses PrintActualOfExpected and keeps its "X of Y rows (P%)" wording.
• src/PlanViewer.Web/PlanViewer.Web.csproj: links the new Core file, because the Web project compiles Core sources directly.
• tests/PlanViewer.Core.Tests/PlanRowAccuracyTests.cs (new): PerformanceMonitor's rule cases and its 2,000-pair sweep, the issue's examples, and the App label and the HTML export for key_lookup_plan.sqlplan. It also checks the source of the Web label, because the test project does not reference the Web project and cannot render that label.

Test Plan

• PlanRowAccuracyTests, NodeLabelRowAccuracyTests and HtmlExporterTests pass on this branch (56 tests).
• The full suite had 0 failures with this change and the [FEATURE] The tiniest modification ever: "Early abort" #613 change together.
• Open tests/PlanViewer.Core.Tests/Plans/key_lookup_plan.sqlplan. The Key Lookup's label reads "1 of 1.128 (89%)".

Generated with Claude Code

https://claude.ai/code/session_019tS6P95Dtzs4aeMpb1xqzX

[claude]

Reviewed the diff. I found nothing blocking.

• PlanRowAccuracy.cs is linked into PlanViewer.Web.csproj, as the repo convention requires.
• It has no dependencies beyond BCL, so it compiles in the Blazor project.
• Extreme inputs are handled: NaN, Infinity and subnormals. The tests pin the cap and the no-exponent behaviour.
• No SQL is generated and no version files are touched.
• The tests cover the App label, the HTML export and the shared formatter. The Web label is only checked by a source-text assertion, which is brittle but acceptable given that Web isn't referenced by the test project.

Minor, non-blocking: WebNodeLabel_UsesTheSharedFormatter will fail on any harmless reformat of that line in Index.razor.