[wolfy-j]

After #75 removes the old traversal depth cutoff, softness detection and return-type scanning still recurse on the Go stack. A standalone graph of 20,000 nested arrays exhausts a deliberately reduced 1 MiB stack budget.

Use traversal-local work lists instead of recursive Go calls. Softness is completed in postorder, preserving cycle rejection and cached results for shared subtrees. Return scanning retains depth-first child order, early matches and predicate-controlled pruning. Existing identity maps still ensure each shared node is traversed once; no depth cutoff or persistent cache is added.

This PR is stacked on #75 and should land after it or be folded into it.

Validation:

• Both reduced-stack subprocess regressions fail before the fix and pass afterward.
• Covers terminal placeholder/concrete evidence, cycles, shared results, scan order and incomplete method declarations.
• Full go test -timeout 25m ./... passes in Linux CI.
• go test ./types/typ ./compiler/check/returns -race -count=1