Green-node flags for fast trivia/content queries

[dsisco11]

Summary
This PR introduces cached bitflags on green AST nodes to enable O(1) checks for trivia/content properties (newline/whitespace/comment/error/keyword/tagged-ident) and to support faster subtree pruning in hot query paths.

Key changes

• Add GreenNodeFlags + mask helpers for boundary vs subtree “contains” semantics: GreenNodeFlags.cs
• Add GreenNode.Flags virtual (default None) so concrete node types can expose computed flags: GreenNode.cs
• Compute flags in leaves from trivia + kind-derived bits (ContainsErrorNode, ContainsKeyword, ContainsTaggedIdent) and cache them: GreenLeaf.cs
• Aggregate flags in containers (GreenBlock, GreenList, GreenSyntaxNode), with boundary flags taken from the first child’s leading boundary + last child’s trailing boundary, and subtree “contains” flags OR’d across children:
  • GreenBlock.cs
  • GreenList.cs
  • GreenSyntaxNode.cs
• (Perf helper) Precompute child offsets for containers with ≥10 children to make GetSlotOffset() O(1): GreenBlock.cs, GreenList.cs

Tests added

• Flags correctness + parsed “ownership” cases for newline/comments: GreenNodeFlagsTests.cs
• Equality/hash stability + cache/non-cache consistency + structural sharing immutability: GreenNodeEqualityTests.cs