Add simplified canonical ordering APIs for FlatExpression

[Copilot]

This explores the canonical-representation direction for FlatExpression by adding ordering primitives directly on ExprTree. The implementation now keeps that support intentionally narrow and simple: it detects canonical post-order directly and rebuilds non-canonical trees through the existing expression round-trip path.

• What this adds

  • Introduces ExprTree.IsInOrder() to detect whether a flat tree is already in canonical post-order.
  • Introduces ExprTree.PutInOrder() to normalize a flat tree into that layout.

• Canonical layout

  • Uses reachable-node post-order:
    • every child appears before its parent
    • the root becomes the last node
    • unreachable nodes are removed
  • This gives a deterministic layout without attempting to reorder semantic child positions for non-commutative expressions.

• Simplified normalization

  • PutInOrder() no longer performs manual node-index remapping, sibling rewiring, closure-constant compaction, or metadata reconstruction.
  • Instead, it rebuilds the canonical flat tree through the existing ToExpression() / FromExpression() round-trip, reducing complexity and LOC while preserving the supported behavior.

• Tests added

  • Covers trees produced from expression conversion that are already canonical.
  • Covers direct helper-built flat expressions that are not canonical, then verifies PutInOrder():
    • removes unreachable helper artifacts
    • produces compact constant indexes
    • preserves round-trip semantics
    • yields the expected node order and child links through explicit index assertions

• Example

  var fe = expr.ToFlatExpression();
  
  if (!fe.IsInOrder())
      fe.PutInOrder();
  
  var roundtrip = fe.ToLightExpression();

[dadhi]

I do not like so much increased complexity and number of locs. Can we simplify back to support only constant indexes and not signal indexes. Let drop dymamic index stuff and thoutougly test all add helpers in assert js

[Copilot]

I do not like so much increased complexity and number of locs. Can we simplify back to support only constant indexes and not signal indexes. Let drop dymamic index stuff and thoutougly test all add he...

Addressed in fdf7b03. PutInOrder() is simplified to rebuild the canonical tree via the existing ToExpression()/FromExpression() path instead of manual index remapping, and the test now asserts explicit helper-produced indexes before and after normalization.

[dadhi]

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