m2-syn 0.4.0

This release replaces the flat generated CST schema with explicit nested
strutuct! declarations and substantially expands semantic visitor dispatch.

Highlights:

• Adds semantic operator, assignment, lambda, member-access, indexing, control,
  and related specialization families.
• Keeps Delimited, Punctuated, and ordinary vectors as transparent storage;
  delimiter boundaries are presented explicitly as Delimiter leaves.
• Adds CellTokenStream, CellTokenTree, and CellTokenGroup for framed parser
  tokens while retaining the common lexical token atoms.
• Treats adjacency as a binary operator and adds explicit augmented operators.
• Canonicalizes Token! punctuation across quoted fragments and arbitrary Rust
  token spacing.
• Adds the ProcMacro interface and LiteralReturnTypes transformation example.

Breaking migration:

• Replace PunctPair<L, P, R> with SeparatedPair<T, P>.
• Do not construct Punctuated for singleton values; it now requires a real
  separator.
• Update generated-node matches, visitors, folds, and constructors to the new
  nested semantic categories.
• Consume framed cells through CellTokenStream and CellTokenTree, projecting
  through to_token_stream or into_token_stream when lexical tokens are
  required.

See CHANGELOG.md for the complete release notes.