test: no test pins the exact boundary of 18 named bounds #1111

Problem

A named bound (maxX or MaxX) caps a walk, a chain or a count. For 18 of the 38 comparisons against one outside test files, moving the comparison one off (> to >=, >= to >, < to <=, <= to <) leaves go test ./... green. A bound documented as N can then admit N−1 or N+1 unnoticed, and a change that moves where a bound cuts cannot show what it moved.

Two examples:

• operation/multipart.go:189, if hops > maxPartRefHops (16). The multipart tests build chains within the bound and past it, and none of them notices the walk cutting one hop earlier.
• schema/discriminator.go:367, depth < maxDiscriminatorAncestorDepth (256). TestAllOf_DiscriminatorValueChainDeeperThanCap straddles the cap at 255 and 260 links, not at the cap and one past it.

The comparisons that survive, under compilers/openapi/internal/ except the last; where a line compares twice, the one against the bound is the one moved:

• annotation/rawjson.go:72: if depth > maxRawDepth
• annotation/unknown.go:250: if len(fresh) > MaxUnknownKeys
• bind/documents.go:268: if d.cfg.Through == nil || passes >= maxPasses
• diag/diag.go:493: for rest := err.Error(); rest != "" && out.Len() < MaxQuotedErrorBytes;
• merge/canonical.go:17: if depth > maxJudgeDepth
• merge/fold.go:47: if len(decls) > maxFoldDeclarations
• merge/typejudge.go:81: if depth > maxJudgeDepth
• merge/typejudge.go:567: if !ok || depth > maxJudgeDepth
• nodeview/nodeview.go:268: if v.cachedPairs+held > maxCachedPairs
• nodeview/nodeview.go:611: for i := 0; n != nil && n.Kind == yaml.AliasNode && i <= maxAliasChain; i++
• oas/engine.go:124: if depth <= MaxDepth && w.budget >= 0
• openapitest/corpus.go:115: if depth >= MaxTreeDepth
• operation/multipart.go:189: if hops > maxPartRefHops
• operation/multipart.go:283: if depth > maxBodyAliasHops
• overlay/sequence.go:263: if depth > maxWeighDepth || w.onPath[n]
• schema/discriminator.go:367: for depth := 0; depth < maxDiscriminatorAncestorDepth && len(level) > 0; depth++
• sourceindex/sourceindex.go:173: if depth <= maxTrackedDepth
• ir/types.go:142: for depth := 0; depth < maxDomainDepth && len(frontier) > 0; depth++

The other 20 redden at least one test when moved one off.

Cause

Boundary tests straddle a bound with a margin instead of sitting on it and one past it, or no committed fixture reaches the bound at all.

Direction

A proposal: give each bound a case at exactly the bound and one past it, reading the constant (through export_test.go where it is unexported) rather than spelling its value. Where the exact value has no observable effect, as for a guard that only has to stop a hostile input somewhere, the bound's comment says so and the site is left unpinned on purpose.

Out of scope

Bounds compared without a max name (literal limits, budgets counted down to zero) were not swept. Tests that pin a diagnostic's wording, including the bound's value it names, are a separate matter: they fail loudly when a bound changes.

Related: #919

Priority: P4, rule 1: a test task that changes no lowering, validation, IR or emit-contract behaviour. Each of the 38 lines git grep -nE '[^=!<>](<=|>=|<|>) ?[a-z]*[mM]ax[A-Z]\w*|[mM]ax[A-Z]\w* ?(<=|>=|<|>)[^=<>-]' -- '*.go' ':!*_test.go' ':!internal/lintrules/testdata' lists, moved one off in turn, then go test -count=1 ./... → 18 green (morphic @49fb350b, PR #1110's head; the 15 files holding those 18 lines are identical on main @ff3fff4d).