fix: preserve non-nil omitempty struct pointers - #601

Fixes #503.

Summary

The OpStructPtrHeadOmitEmpty VM path dereferenced a struct pointer and then fell through to the struct-value omit-empty path. That path could interpret a zero-valued first child field as a nil pointer and omit the entire non-nil child.

This change keeps pointer traversal and nil checks at the correct indirection depth in the VM template, then regenerates all four VM variants (vm, vm_color, vm_indent, and vm_color_indent). For direct-interface root structs, the compiler consumes exactly the pointer layer already held in the interface word, preserves additional pointer depth and recursive layouts, and bases recursion checks only on encoded fields. The regression matrix covers zero-valued first fields, nil and multi-level pointers, nested and sibling fields, pointer-valued child fields, ignored markers and back-references, and recursive roots across plain, colorized, indented, and colorized-indented output.

Testing

• go test . -run '^TestIssue503$' -count=100 with Go 1.21.13
• go test . -run '^TestIssue503$' -count=10 with Go 1.19.13 and 1.20.14
• go test ./... -count=1 with Go 1.19.13, 1.20.14, and 1.21.13
• go test -race ./... -count=1 with Go 1.21.13
• go generate ./internal/... with no generated diff
• golangci-lint v1.54.2 run --timeout=5m
• go vet ./... reports exactly the same existing diagnostics as master