Plan viewer: show each warning's actionable fix, and take severity colours from one helper

[erikdarlingdata]

Fixes #4572, part of #4511.

Why

PerformanceStudio's viewer (erikdarlingdata/PerformanceStudio@3d717e7) closed a gap where a
warning's MaxBenefitPercent and ActionableFix were computed but not rendered everywhere the
warning itself was shown. PM's properties panel already rendered the benefit percentage in the
warning header, but never read ActionableFix, so a warning with a concrete fix suggestion showed
only its generic message.

Separately, PerformanceStudio (@51d59ca) replaced four copies of the same
Critical ? "#E57373" : Warning ? "#FFB347" : "#6BB5FF" ternary with one
WarningSeverityBrush(severity) helper. PM had the identical duplicated pattern, five times across
PlanViewerControl.Properties.cs and PlanViewerControl.Tooltips.cs.

What changes

• PerformanceMonitor.PlanAnalysis.PlanWarningDisplay gets a new pure helper,
  WarningSeverityColorHex(PlanWarningSeverity), returning the same three hex values PM already
  used. No colour value changed — this is pure de-duplication, matching PerformanceStudio's shape
  going forward.
• All five inline ternary sites (two in Properties.cs, two in Tooltips.cs, previously also
  duplicated a third time in Properties.cs) now call the helper instead of repeating the
  literal.
• Both warning sections in PlanViewerControl.Properties.cs (plan-level warnings and per-node
  warnings) now add an italic TextBlock for w.ActionableFix under the warning message, shown
  only when the field is non-null/non-empty, mirroring PerformanceStudio's Avalonia pattern
  (Margin = new Thickness(16, 2, 0, 0), FontStyle = Italic, same foreground as the message
  line). The tooltip warning list (Tooltips.cs) keeps its existing shorter text — PerformanceStudio
  never added an ActionableFix line there either, only the colour-helper change touches that file.

Lite: gets this too — PlanViewerControl.Properties.cs/Tooltips.cs are in the shared
PerformanceMonitor.Ui project, hosted by both the Darling Viewer and Lite.

PM-ahead / not ported: none identified for this issue's scope.

Nothing user-visible yet. No analyzer rule in PerformanceMonitor sets ActionableFix today (every assignment in PerformanceMonitor.PlanAnalysis is null), and neither does PerformanceStudio dev (its only assignment, in BenefitScorer, is null). So the new italic line renders for no current warning; it is in place for when a rule gains a fix. The colour change is de-duplication with identical values. The italic line is WPF layout with no headless test surface, and with no warning carrying a fix there is nothing to screenshot yet.

Test plan

New file Darling/Darling.Tests/PlanViewerWarningColorAndFixTests.cs:

• WarningSeverityColorHex_Critical_IsRed / _Warning_IsAmber / _Info_IsBlue — pin the three
  colour values through the new helper. RED on dev: compile failure
  (CS0117: 'PlanWarningDisplay' does not contain a definition for 'WarningSeverityColorHex'),
  confirmed by building this test file against origin/dev in a detached worktree.
• PlanWarning_WithActionableFix_CarriesTheFixText / _WithoutActionableFix_IsNull — sanity pins
  on the existing model field (not new; documents the contract the rendering code now reads).
• PlanViewerControl_HasNoInlineSeverityColorLiterals — a census pin: no #E57373/#FFB347/
  #6BB5FF literal remains in any PlanViewerControl.*.cs file. Runtime RED on dev, confirmed
  the same way (this assertion built cleanly against dev and failed with Assert.DoesNotContain() Failure: Sub-string found — the old ternaries are still there).

Build (all 0 warnings / 0 errors, Release, -p:EnableWindowsTargeting=true):
Darling.Tests, PerformanceMonitor.Ui, Lite/PerformanceMonitorLite.csproj, Lite.Tests,
Darling/PerformanceMonitor.Darling.Viewer.

Run, in-process on macOS (Microsoft.WindowsDesktop.App stripped from the runtimeconfig):
the new class, every PlanSync*/Viewer* class and the plan-analysis classes:
Total: 1810, Errors: 0, Failed: 86, Skipped: 137. All 86 failures are pre-existing
System.IO.FileNotFoundException: PresentationFramework (and one TypeInitializationException
wrapping the same) platform failures in classes this change doesn't touch (ViewerDrillDownTests,
ViewerChartContextMenuTests, ViewerHistoryWindowTests, etc. — WPF-type-dependent tests that
can't load PresentationFramework on macOS). None of the failing test
names are in a PlanViewer* file. The macOS-runnable subset (the new class, the PlanSync* classes and the plan-analysis classes) ran green at Total: 249, Errors: 0, Failed: 0, Skipped: 2.

Lite.Tests builds (0/0) but cannot run on macOS (discovery dies on WindowsBase); this change adds
no new Lite.Tests file since the touched files are shared through PerformanceMonitor.Ui and
already covered by the Darling.Tests pins above. CI decides Lite/Darling.Tests-on-Windows and the
WPF-dependent classes.

Screenshot plan (pure layout, for a human on Windows):

1. Open a captured plan with at least one warning that has a non-null ActionableFix (any serial-
   plan or missing-index warning the analyzer scores). In the Properties panel's "Plan Warnings" or
   node "Warnings" section, confirm an italic line appears under the warning message, indented to
   match the message (not the header), in the same muted foreground colour as the message text.
2. Confirm a warning with no ActionableFix shows no extra line (no blank italic row).
3. Confirm the warning header colour (critical/warning/info) is visually unchanged from before this
   PR in both the properties panel and node tooltips — the colour helper returns the same hex
   values, so there should be no difference to see.
4. Repeat in both the Darling Viewer and Lite (shared control).

CHANGELOG

None: no user-visible change yet. The fix line renders only for a warning that carries an ActionableFix, and no rule sets one today; the severity colours are unchanged.