openapi: raw YAML tree walks each carry their own stack, depth cap and alias rule #788

Problem

The compiler walks raw yaml.Node trees in many places, and each walk carries its own stack or recursion, depth cap and decision about aliases. At 210283f, among others:

• sourceindex/sourceindex.go walk: iterative, explicit frame stack, ancestor set, node bound; does not follow aliases.
• load/external.go holdSpellings: iterative, explicit stack, follows an alias only to read a $ref's key and value.
• overlay/overlay.go substituteGrafts and expandAlias: recursive, counting down a budget passed by pointer.
• load/load.go walkNumericScalars: recursive with a depth parameter.
• internal/harness/order.go reverseNode: recursive with maxReverseDepth.

Each had to decide the same four things: how deep, how many nodes, whether an alias is an edge, and what to do when a bound is hit. Where the answer is "iterative with an explicit stack" it is re-derived each time, and the bounded-recursion rule is easiest to get wrong exactly here: untrusted input chooses the depth.

The converters value.fromNodeAt and rawConv.node also recurse with depth caps, but they build a result bottom-up and are not walks; they stay as they are.

Proposed direction

A small iterative walker in compilers/openapi/internal taking a root, a depth bound, a node bound (the counter from #785) and an alias policy (follow or skip), calling a visitor with the node and its depth and letting it prune. Migrate the walks whose semantics match; leave those that are really searches over a different graph (reach, scan) alone.

Keeps to compilers/openapi/internal; the framework-promotion rule (docs/micro-compiler-design.md §3) applies if another compiler shows the same need.

Acceptance

• Each migrated walk loses its private stack and bound; its existing bound tests pass.
• The walker reports truncation to the caller rather than stopping silently, as sourceindex.Index.Truncated does.
• Deep (thousands of levels) and wide inputs are covered by a test at the walker, not once per caller.
• Goldens and the harness sweep unchanged.