fix: harden silence processing, API, packaging, and tests

[BTawaifi]

Summary

• compute silent ranges as the full complement of normalized non-silent ranges
• preserve trailing silence and fully silent files
• return a structured processing result and let Python callers receive failures
• make CLI failures return non-zero status
• derive collision-resistant output names from the input and support configurable output formats/stems
• modernize package build metadata, Python support declaration, and package exports
• exclude tests from built distributions
• replace mock-only coverage with 26 regression tests including real pydub tone/silence detection
• expand CI across Python 3.10–3.13 and verify built wheel contents/API/CLI

This PR is intentionally broad because it closes the correctness and verification gaps found in a full code review.