feat(traverse): mutation-capable traversal — MutablePath with replaceWith/remove/insert/skip/stop - #324

Summary

Stage 2 of the AST convergence plan (constructive-planning #1323): a principled, Babel-inspired mutation API for @pgsql/traverse, added as a new traverse(root, visitor) entry point alongside the untouched read-only walk/visit.

traverse(ast, {
  RangeVar(path) { path.replaceWith({ RangeVar: {...} }); },
  ResTarget: { enter(path) {...}, exit(path) {...} },
  enter(path) { /* catch-all */ }
});

MutablePath extends the existing NodePath shape (tag / node / parent / keyPath) with the container slot (container, containerKey) that physically holds the node, enabling:

• replaceWith(value) — writes container[key] = value; the replacement's children are traversed but the visitor is not re-invoked on the replacement itself (no self-requeue → a visitor replacing a node with the same tag cannot loop)
• remove() — splices from array containers / deletes object fields; children not traversed; sibling array iteration stays aligned (iterators consume the net length delta reported by each visit)
• insertBefore(...) / insertAfter(...) — array containers only; inserted values are not visited
• skip() / return false — skip children (same semantic as walk)
• stop() — halt the whole traversal via a shared TraversalState
• {enter, exit} handlers per tag plus catch-all enter/exit; exit runs post-order

Traversal is the same runtime-schema-driven walk as walk — tagged nodes, tag-synthesized concrete typed fields (e.g. CreatePolicyStmt.table: RangeVar stored bare), untagged ParseResult/ScanResult roots — so mutation works uniformly on both storage conventions (replaceWith takes the value exactly as stored: tagged wrapper or bare node data). Mutating the detached root throws.

Deliberately no lexical scope/binding layer — SQL object identity lives in the semantic object graph (planning #1322), not the traversal.

14 new tests in mutate.test.ts; 40/40 green in the package.

Link to Devin session: https://app.devin.ai/sessions/025fb88043964fdbb335ac5e39df2478
Requested by: @pyramation