em 01: scaffold ExtractMethod tool + Demo fixture

- tools/ExtractMethod: net10.0 console app (Microsoft.CodeAnalysis.CSharp
  5.9.0, pinned latest stable). CLI: <file.cs> <startLine> <endLine>.
  Parses the file, builds a scratch compilation with refs from
  TRUSTED_PLATFORM_ASSEMBLIES, and reports how many whole statements the
  line range covers (clean error otherwise; exit codes: 0 ok, 1
  resolution error, 2 usage).
- Tooling/CompilationLoader: shared parse + compilation path for CLI and
  tests (tests exercise the exact loading path the CLI uses).
- Tooling/SelectionResolver: snaps a 1-based inclusive line range to
  whole statements in the enclosing method body block; boundary checks
  never split a statement; nested/blank-line ranges handled cleanly.
- tests/: ExtractMethod/Fixtures/Demo.cs checked-in fixture exercising
  every bucket of the parent spec (read-only local, written+read-later
  return, scratch local, param read, field+property access, indexer +
  method invocation, multi-statement range incl. a for-loop); excluded
  from project compilation, copied to output as data.
- Tests: Demo.cs compiles with no diagnostics; range 68..72 resolves to
  2 statements (LocalDeclarationStatement, ForStatement); AnalyzeDataFlow
  succeeds on the fixture's for-loop node; mid-statement range fails
  cleanly. 30/30 green.

.gitignore

@@ -3,4 +3,6 @@ artifacts/
 bin/
 obj/
 .dotnet-cli/
-*.user
+*.user
+# NuGet restore scratch dir (DOTNET_CLI_HOME fallout)
+.local/