em 03: extract-first scan + full suggestion report

- ExtractFirstScanner: reads-only scan of non-local/param read expressions
  (member/field/property access, element access, invocation, cast), deduped
  by symbol+text with occurrence counts; invocations flagged (hoisting
  changes eval count); own-class field/property reads marked optional.
  Assignment LHS skipped whole (decision 5) — surfaced as a report note.
- SignatureBuilder: promotes bucket overlaps into one coherent signature
  (declared-inside vars drop from params; declared-inside return candidates
  become the return; outside candidates already covered by their ref
  write-back stay ref params). Trailing composite returns stay void with a
  note suggesting an extract-variable first.
- ExtractionReporter + ReportFormatter: single composition shared by CLI and
  tests; report = header, params (ref), returns, locals, extract-first
  (count + flags), notes, suggested-signature line last.
- DataFlowClassifier: Returns now carry types (ReturnSuggestion) — the
  signature line needs them.
- Displays: compact symbol/type formatting for the report.
- Fixture: RepeatReads (dedupe ×2 + skipped field write), Casts (cast
  candidate) appended at the end so all pinned line numbers stay put.

tools/ExtractMethod/Tooling/Displays.cs

@@ -0,0 +1,53 @@
+using System.Collections.Immutable;
+using Microsoft.CodeAnalysis;
+
+namespace ExtractMethod.Tooling;
+
+/// <summary>
+/// Compact symbol display for the suggestion report. The defaults are wrong
+/// for a report a human reads next to their own source:
+/// - <c>ToDisplayString()</c> fully qualifies every namespace
+///   (<c>System.Collections.Generic.List&lt;Widget&gt;</c>),
+/// - <c>ToMinimalDisplayString()</c> keeps containing types AND prefixes
+///   members with their own type (<c>int Demo._seed</c>).
+/// The report wants the shortest form that stays readable in context:
+/// types as bare names (<c>Widget</c>, <c>List&lt;Widget&gt;</c>), members as
+/// <c>ContainingType.Member</c> with parameter types.
+/// </summary>
+internal static class Displays
+{
+    /// <summary>
+    /// Type display for suggestion lines: shortest unambiguous-enough form
+    /// (no namespaces, no containing types, keyword spellings for special
+    /// types, nullable annotations kept). Built explicitly because
+    /// MinimallyQualifiedFormat keeps containing types for nested types
+    /// (<c>Demo.Widget</c>) and has no With-style override here.
+    /// </summary>
+    internal static readonly SymbolDisplayFormat TypeFormat = new(
+        globalNamespaceStyle: SymbolDisplayGlobalNamespaceStyle.Omitted,
+        typeQualificationStyle: SymbolDisplayTypeQualificationStyle.NameOnly,
+        genericsOptions: SymbolDisplayGenericsOptions.IncludeTypeParameters
+            | SymbolDisplayGenericsOptions.IncludeVariance,
+        miscellaneousOptions: SymbolDisplayMiscellaneousOptions.UseSpecialTypes
+            | SymbolDisplayMiscellaneousOptions.EscapeKeywordIdentifiers
+            | SymbolDisplayMiscellaneousOptions.IncludeNullableReferenceTypeModifier);
+
+    /// <summary>Compact member display for extract-first entries:
+    /// <c>Demo._seed</c>, <c>Demo.Scale</c>, <c>Widget.Bigger(Widget)</c>,
+    /// <c>List&lt;Widget&gt;.this[int]</c>.</summary>
+    internal static string Member(ISymbol symbol) => symbol switch
+    {
+        IMethodSymbol method =>
+            $"{Type(method.ContainingType)}.{method.Name}({Parameters(method.Parameters)})",
+        IPropertySymbol { IsIndexer: true } indexer =>
+            $"{Type(indexer.ContainingType)}.this[{Parameters(indexer.Parameters)}]",
+        IFieldSymbol or IPropertySymbol or IEventSymbol =>
+            $"{Type(symbol.ContainingType)}.{symbol.Name}",
+        _ => symbol.ToDisplayString(TypeFormat),
+    };
+
+    private static string Parameters(ImmutableArray<IParameterSymbol> parameters)
+        => string.Join(", ", parameters.Select(p => Type(p.Type)));
+
+    private static string Type(ITypeSymbol? type) => type?.ToDisplayString(TypeFormat) ?? "?";
+}