fix: make qtest multi-input order deterministic

[tiye]

Summary

• globally sort all files discovered by qtest across multiple inputs
• deduplicate repeated file arguments so each test file runs once
• add TAP regression coverage and clarify RFC 0079

Validation

• make check
• focused TAP/JSON CLI tests
• git diff --check

This PR is chained after PR #35 and is intentionally not merged automatically.

[Copilot]

Pull request overview

This PR makes qtest’s handling of multiple input paths deterministic by normalizing the discovered file list (global sort + dedup), and extends TAP-related regression coverage while updating the TAP RFC to match the new contract.

Changes:

• Sort and deduplicate the aggregated set of discovered test files before execution/output.
• Extend CLI TAP regression test coverage to validate determinism with reversed/duplicate inputs.
• Update RFC 0079 to specify global sort + dedup semantics for TAP mode.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/cli_tools.rs	Adds TAP regression assertions for deterministic ordering and dedup with multiple file inputs.
src/bin/qtest.rs	Globally sorts and deduplicates collected file paths to make multi-input runs deterministic.
RFCs/0079-qtest-tap-output.md	Updates TAP CLI contract documentation to reflect global sort + dedup behavior.

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