irverify: run every walking check from one traversal of the document #962

Problem

irverify.Verify walks the whole document 17 times: 16 ir.WalkValues walks and one ir.ValueCycles. On the golden spec at c387e25c (the Cloudflare API schema, cloudflare/api-schemas openapi.json at 911ad78, compiled to 41,592 types), it makes:

• 121.8 M visits;
• 132.9 M allocations;
• 17.1 GiB allocated;

and takes about 7.4 s, longer than the 7.1 s compile. The 17 walks cost 0.4 to 0.5 s each, so no single check dominates. Path-string building (fieldPath) is about half of the walk's CPU (#813). Verify runs in every harness sweep and corpus test, so this is test and CI time.

Direction

A proposal, measured on a throwaway prototype:

• Each walking check becomes a handler over the nodes it cares about (kinds, visit, cycle, finish).
• A 64-line dispatcher runs all of them from one traversal.

Across the 20 largest corpus documents, allocations fell 13 to 15×. The output was identical, []Violation equal under cmp.Equal, on all 1,839 compiled documents and in 3,000 trials that planted random defects into 60 of them. The copied irverify suite passes. Dropping any one handler reddens it.

Two costs:

• The change adds 74 code lines (about 5%).
• Cycle detection needs a cycle callback on the ir walker, about 8 lines.

The 10 checks that loop over registries rather than walking stay as they are. pass.Validate's 3 walks could share the same traversal later; that is not measured.

Done when Verify makes one traversal, with a counted-work test pinning it.

Out of scope

Merging irverify into pass.Validate. Only 7 of Validate's 17 checks overlap, the saving is in the traversal rather than the package boundary, and the two report on different channels (our bugs versus spec problems).

Related: #813, #859

Priority: P4, rule 1 (a refactor that changes no output). Count ir.WalkValues calls during one irverify.Verify of golden.ir.json → 16, plus one ir.ValueCycles (spec @911ad78, morphic @c387e25c).