Provide one canonical frontend AST traversal interface

[arniber21]

What to build

Add a canonical exhaustive AST traversal interface in the frontend and migrate read-only tooling walkers to it. Future AST variants should require one traversal update rather than independent hand-written recursion in the linter and language server.

Acceptance criteria

• The frontend exposes exhaustive traversal for declarations, expressions, statements, patterns, types, and type-level expressions.
• Traversal supports pre/post hooks and preserves source spans.
• quonlint and applicable quon_lsp intelligence features use the shared traversal.
• Formatting keeps only the structural recursion needed for layout decisions and shares traversal helpers where semantics permit.
• A synthetic new-node test demonstrates that nested nodes are visited consistently by migrated consumers.

Blocked by

None - can start immediately.

[arniber21]

Implemented and merged via PR #448. Closing as complete.