Support xor groups with positional arguments

[ColumbusLabs]

Fixes #255.

This validates xor groups against both flags and positional arguments owned by the same application or command node. A set positional can also satisfy required flags in its XOR group.

Grouping directly from each node keeps command scopes independent: command-local conflicts are rejected, while a root flag and child positional may reuse the same group name without conflicting.

Tests cover root and command positionals, single-side success, cross-scope isolation, and required-XOR satisfaction.

Verification:

• go test ./...
• go test -race ./...
• ./bin/golangci-lint run
• gofmt / git diff --check

[alecthomas]

@codex review

[chatgpt-codex-connector]

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[ColumbusLabs]

Addressed in 49c114d after rebasing onto current master. Required positional XOR alternatives now satisfy both missing-positional validation paths when a peer flag is set, and XOR/AND overlap validation compares value identity rather than display names. Added regressions for both cases. Verified with go test ./..., go test -race ./..., and ./bin/golangci-lint run.