fix(gnovm): depth-based shadowing for promoted struct fields and methods

[omarsy]

Description

StructType.FindEmbeddedFieldType treated any second embedded match as a conflict, with no notion of promotion depth. So a method or field promoted from a shallower embedded field did not shadow a same-named one reachable deeper, and gno reported a spurious conflict and rejected valid programs:

type Deep struct{};    func (Deep) M() {}
type Mid struct{ Deep }              // M at depth 2
type Shallow struct{}; func (Shallow) M() {}
type T struct { Shallow; Mid }       // Go: T.M resolves to Shallow.M

gno rejected var i I = T{} at preprocess with T does not implement I (missing method M). On-chain addpkg type-checks with go/types first (which accepts these), so such a package passed the deployment gate and then panicked at VM preprocess.

Change

Search direct (depth-0) fields first (a direct field shadows promotion), then keep the shallowest embedded candidate (trail length = depth); two at the same shallowest depth are ambiguous. The recursion-guard set and the 5-value return contract are unchanged, so runtime selector navigation and all callers are unaffected.

Scope (important)

This closes the common disjoint-subtree shadowing case. It is a minimal, zero-regression step (verified by differential fuzzing vs. the Go toolchain: 0 regressions over 400 random shapes). It does not fix the broader pre-existing promotion bug where the shared recursion-guard set prunes legitimate paths when the same type is reachable via multiple embedded paths — tracked in #5819, which also covers the diamond false-accept. The ADR documents both out-of-scope cases.

Tests

embed_method_shadow0/1/2 pin the fix (they panic at preprocess on the old code); embed_method_shadow_ambiguous, embed_method_shadow_fieldmethod, and embed_method_shadow_internal_ambiguous are parity/regression guards (pass on old + new), with go/types parity via TypeCheckError directives. Confirmed against the Go toolchain across shadowing, multi-level, pointer-embedding, same-depth ambiguity, internally-ambiguous siblings, self-cycle, and embedded-interface cases.

Note

Draft. Local full-TestFiles suite passes (0 failures); CI will also run it. This PR is AI-assisted; ADR included per AGENTS.md (gnovm/adr/prxxxx_embedded_method_depth_shadowing.md, to be renamed with the PR number).

Related spec-compliance fixes from the same effort: #5784, #5785, #5807, #5808.

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