Engine tests: manual change, config edits and app start rows of table 6.4

[janthoXO]

Tests/KlangLadderCoreTests/RulesTests.swift covers Rules.target and the ScopeConfig operations. The rows of DESIGN.md table 6.4 that live in Engine have no tests:

• The user changes the default manually: no action (G6).
• The user reorders, enables, disables or deletes: no action.
• First app start in a login session: switch to the best enabled device (G20).
• Later app start in the same session: no action.
• A late macOS auto-switch after the device-list evaluation (defaultChanged with recentConnect).
• Delete of a device that reconnected in the meantime is rejected (5.1 race).
• Very first run seeds the current default at Homebrew tap: formula that builds from source #1.

Blocker: Engine uses the concrete CoreAudio struct, ConfigStore and LoginSession directly. To test these rows, inject a small fake audio source (devices, default UID, setDefault) and a config/session location.