Tighten FlatExpression naming and 16-bit index storage

[Copilot]

FlatExpression had drifted in two places: comments were noisy/inconsistent, and index naming/storage conventions were not enforced uniformly. This change trims the file back to a single Idx vocabulary and makes the 16-bit storage contract explicit in code.

• Comment cleanup

  • Rewrote FlatExpression.cs comments to plain technical prose.
  • Removed non-ASCII symbols and overly verbose/LLM-like phrasing.
  • Kept comments focused on layout, invariants, and identity semantics.

• Naming normalization

  • Renamed the root field from RootIndex to RootIdx.
  • Normalized local/parameter naming across the file to Idx/Idxs where the file already used that convention.
  • Updated affected tests and property-test helpers to match the new API.

• 16-bit index enforcement

  • Replaced implicit truncation with checked((ushort)...) when writing child/next metadata into packed node fields.
  • Preserves the existing compact layout while failing fast if a stored index exceeds the intended 16-bit range.

• Focused regression coverage

  • Added a test that locks in the compact metadata sizes:
    • ExprNode remains 24 bytes
    • LambdaClosureParameterUsage remains 6 bytes
  • Added an overflow boundary check to verify that exceeding the 16-bit node-index range throws instead of silently wrapping.

var fe = default(ExprTree);
for (var i = 0; i < ushort.MaxValue; ++i)
    fe.Default(typeof(int));

Asserts.AreEqual(ushort.MaxValue, fe.Nodes.Count);
Asserts.Throws<OverflowException>(() => fe.ParameterOf<int>("overflow"));

[dadhi]

ci build is failing:
Error: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs(781,34): error CS0103: The name 'Unsafe' does not exist in the current context [/home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.LightExpression.UnitTests/FastExpressionCompiler.LightExpression.UnitTests.csproj::TargetFramework=net472]
Error: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs(782,33): error CS0103: The name 'Unsafe' does not exist in the current context [/home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.LightExpression.UnitTests/FastExpressionCompiler.LightExpression.UnitTests.csproj::TargetFramework=net472]

[Copilot]

ci build is failing:
Error: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/test/FastExpressionCompiler.LightExpression.UnitTests/LightExpressionTests.cs(781,34): error CS0103: The nam...

Fixed in 15012aa. I replaced the Unsafe.SizeOf calls with a net472-compatible layout size check, then verified the affected project builds with dotnet build test/FastExpressionCompiler.LightExpression.UnitTests/FastExpressionCompiler.LightExpression.UnitTests.csproj -f net472 and the main net9.0 test runner still passes.