Optimize FlatExpression node layout and child reuse semantics

[Copilot]

FlatExpression was still paying unnecessary space and construction costs: constants used an 8-byte in-place payload, child links only modeled sibling chains, and repeatedly used nodes were eagerly cloned instead of being reused until aliasing actually occurred. This change tightens the node layout and shifts reuse/linking behavior toward first-use ownership plus explicit reference nodes on subsequent reuse.

• ExprNode layout

  • split the old 8-byte payload into:
    • uint _data for inline constant payload or ChildIdx/ChildCount
    • uint _meta for NodeType, flags, and encoded NextIdx
  • kept ExprNode at 24 bytes while restoring direct NextIdx + NodeType storage
  • encoded NextIdx with a non-zero sentinel so index 0 remains valid

• Constant representation

  • inline small 4-byte constants directly in node storage
    • bool, char, byte/sbyte, short/ushort, int/uint, float, and enums with those underlying sizes
  • leave larger values (for example long) boxed/out-of-line
  • retain closure-backed storage for non-inline object constants

• Child linking / reference model

  • stop cloning nodes on first link
  • first parent sets the node’s NextIdx
  • subsequent parents get a lightweight reference node whose ChildIdx points to the declaration node
  • last child in a chain now points back to the owning parent, enabling upward traversal

• Identity preservation

  • parameter and label flattening now reuses the original declaration node and emits reference nodes only when reused
  • reader resolves reference nodes transparently, preserving parameter/label identity on roundtrip

• Coverage

  • added focused assertions for:
    • inline constant packing
    • reference-node creation on child reuse
    • semantic equivalence of reused-parameter expressions after roundtrip

Example of the new reuse behavior:

var fe = default(ExprTree);
var p = fe.Parameter(typeof(int), "p");
var sum = fe.Add(p, p); // first use keeps p, second use becomes a ref node
fe.RootIndex = fe.Lambda<Func<int, int>>(sum, p);

This keeps the original declaration node as the primary representation and only materializes an extra node when reuse requires alias-safe linking.

[dadhi]

Ci Error: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/src/FastExpressionCompiler.LightExpression/FlatExpression.cs(1284,16): error CS0103: The name 'Unsafe' does not exist in the current context [/home/runner/work/FastExpressionCompiler/FastExpressionCompiler/src/FastExpressionCompiler.LightExpression/FastExpressionCompiler.LightExpression.csproj::TargetFramework=netstandard2.0]

[Copilot]

Ci Error: /home/runner/work/FastExpressionCompiler/FastExpressionCompiler/src/FastExpressionCompiler.LightExpression/FlatExpression.cs(1284,16): error CS0103: The name 'Unsafe' does not exist in the c...

Fixed in d4888d2 by replacing the Unsafe float bit-casts with an explicit-layout uint/float helper that builds on netstandard2.0. I also verified the library builds for netstandard2.0 and net8.0 and re-ran the LightExpression test harness successfully.

[dadhi]

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