feat: modernization phase 5 PR4

[dsmurfin]

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[Copilot]

Pull request overview

This PR continues the Phase 5 modernization work by replacing the legacy transmitCounter-driven keep-alive cadence with independently tracked keep-alive tick counters for levels (NULL) and per-address priority (PAP), and makes both intervals configurable on sACNSource.

Changes:

• Replace SourceUniverse.transmitCounter with ticksSinceLevels / ticksSincePriorities and corresponding reset/advance APIs.
• Add configurable keep-alive intervals to sACNSource (defaults 800 ms NULL / 1000 ms PAP) and drive buildDataMessages() keep-alives via tick thresholds.
• Update/extend unit tests to characterize the new keep-alive behavior and configurability.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Tests/sACNKitTests/SourceUniverseTests.swift	Updates state-machine tests for the new keep-alive tick counters and reset behavior.
Tests/sACNKitTests/SourceTransmitTests.swift	Updates keep-alive cadence tests and adds coverage for configurable/clamped keep-alive intervals.
Sources/sACNKit/Source/SourceUniverse.swift	Replaces transmitCounter with separate keep-alive tick counters and reset hooks.
Sources/sACNKit/Source/sACNSource.swift	Adds keep-alive interval configuration and updates transmit builder logic to use tick-based thresholds.
.claude/rules/timing.md	Updates timing documentation to reflect the new default/configurable keep-alive behavior.

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