feat(traverse): dispatch on bare ParseResult/ScanResult roots + payload guards

[pyramation]

Summary

Follow-up robustness pass on #313. libpg-query's parseSync() returns the top level as a bare {version, stmts} object — no {ParseResult: {...}} wrapper — so walk(parseSync(sql), visitor) fell into the generic untagged branch and the ParseResult/RawStmt visitors never fired (visit() was worse: it took the first key version as the tag and traversed nothing).

// walk()/visit(): at the true root only (parent === null, empty keyPath)
detectUntaggedRootTag(root) {
  if (Array.isArray(root.stmts)  && typeof root.version === 'number') return 'ParseResult';
  if (Array.isArray(root.tokens) && typeof root.version === 'number') return 'ScanResult';
  return null;
}

When detected, the root is walked as that node via the runtime schema, so ParseResult → stmts: RawStmt[] (untagged elements) → each statement all dispatch correctly:

walk(parseSync('CREATE POLICY ...'), { ParseResult, RawStmt, RangeVar }) // all fire now

Also hardened walkNode to ignore tagged wrappers whose payload isn't an object (e.g. {Weird: 'str'} previously iterated string indices via for..in).

Detection is deliberately root-only — nested {version, stmts} shapes are untouched (covered by a test). Tagged roots and all other behavior are unchanged. This also benefits plpgsql-parser's walkSqlExpressions path, which passes hydrated parseResult objects straight to walkSql.

Tests

New cases in packages/traverse/__tests__/typed-fields.test.ts for both walk and visit: bare parse-result root dispatch, non-root {version, stmts} not misdetected, non-object tagged payload ignored. Full workspace build + tests pass (traverse 29, transform 1437, deparser 714, plpgsql-parser 303, etc.).

Link to Devin session: https://app.devin.ai/sessions/0ce0d0a13c624fb8b8eb22c77d448fb6
Requested by: @pyramation

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