fix(gnovm): meter GC traversal of large primitive-keyed maps

[omarsy]

Fixes #5883.

Problem

GC visit accounting undercharged the O(N) traversal of a fully-primitive map (map[int]int etc.). MapValue.VisitAssociated walks the MapList in O(N), but the visit counter (GCVisitorFn) only increments when vis is called on a boxed key/value object. For map[int]int both TypedValue.V are nil (data lives in .N), so vis is never called for the entries — an N-entry map counted as ~1 visit and gcVisitGas charged the ~29-gas flat minimum for an O(N) traversal.

Fix

When visiting a MapValue, count one visit per entry whose key and value are both unboxed (Key.V == nil && Value.V == nil) — exactly the entries that otherwise contribute zero. Boxed keys/values are already counted when vis reaches them, so object/mixed-keyed maps are unaffected (no gas change). Deterministic (walks MapList in insertion order, not the Go vmap).

Test

gnovm/pkg/gnolang/gc_map_visit_test.go: a 1000-entry map[int]int now yields visitCount = 1001 (was 1); gcVisitGas 40040 (was 29). Verified red→green (fails on unfixed code).

Verification (consensus-gas change)

• go build ./gnovm/... + go vet + gofmt clean.
• ./gno.land/pkg/sdk/vm -run Gas, full ./gno.land/pkg/integration -run TestTestdata (txtar), ./gnovm/pkg/gnolang -run Files -test.short — all green. Object/mixed maps get +0, so no gas goldens shift.
• Adversarially reviewed across 14 map types (map[int]int/bool/string, map[[1]int]int, map[int]*S, slice/struct/interface values, nested, interface{} holding primitive-vs-pointer): fully-unboxed entries counted, boxed never double-counted; overflow-safe (~9 orders of magnitude headroom).

Note

A fully-primitive map is list-walked twice per GC (once to count, once in VisitAssociated). Kept surgical to avoid changing gas for object maps; a single-walk form would need threading the count through the Visitor signature — happy to adjust if you'd prefer that.

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[Gno2D2]

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