feat: modernization phase 5 PR5

[dsmurfin]

No description provided.

[Copilot]

Pull request overview

This PR (Modernization Phase 5 PR5) adds regression lock-in for a couple of already-correct on-wire behaviors in the source transmit path, plus accompanying documentation updates for Phase 5 completion.

Changes:

• Add transmit-builder regression tests covering (1) NULL+PAP sequence consumption in a single frame and (2) universe-discovery reserved-field zeroing and universe-list sorting.
• Add an internal sACNSource test seam to rebuild and return universe-discovery pages for unit testing.
• Document Phase 5 completion/status and PR5’s specific coverage additions.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
Tests/sACNKitTests/SourceTransmitTests.swift	Adds new regression tests for sequence numbering and universe-discovery packet layout.
Sources/sACNKit/Source/sACNSource.swift	Adds internal buildUniverseDiscoveryMessages() seam to support unit testing discovery packet layout.
MODERNIZATION.md	Updates Phase 5 status summary to indicate completion and references Phase 5 doc.
docs/modernization/phase-5.md	Records PR5 delivered note describing the specific test gaps filled and verification status.

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