Integrate feature set and harden Bungee audio

[0cwa]

Summary

This PR integrates the selected feature work from 0cwa/mixxx#14 on top of the PR20 head:

• memory-cue creation, navigation, editing, waveform marks, controller mappings, and Rekordbox import;
• the narrow Rekordbox USB key-sorting/schema fix, while preserving PR20's existing import guards;
• waveform downbeat and 16-beat markers with the related beat-grid and LateNight skin updates;
• a Bungee low-latency audio-cutout fix that preserves the ReadAhead cursor and loop/jump transaction across asynchronous cache misses;
• atomic retry staging and safe Bungee reset/flush handling;
• CMake BUNGEE defaulting to ON, so CI/CD configurations without an explicit override build Bungee.

The branch is based on PR20 commit 045eecd2bf354c9b9be7a21a93854f96ed1c83cb and integrates the feature commits selectively because PR14's current head conflicts with PR20.

Local validation

• Native mixxx-build target build: cmake --build build --target mixxx-test --parallel 2 — passed.
• Focused native feature coverage: BeatsTest.*:EngineSyncTest.* — 95 tests passed (one disabled).
• Flatpak Builder packaging in mixxx-build with the Mixxx Flatpak manifest and Qt 6.10/Bungee — all 1,137 build targets compiled and the application exported successfully.
• Bungee-enabled mixxx-test build with ccache disabled — passed.
• Focused Bungee/ReadAhead/CachingReader run — 38/38 tests passed, including retry transaction and seek-cancellation regressions.
• git diff --check — passed.

Known validation blocker

The full Flatpak CTest phase previously ran 1,260 enabled tests: 1,258 passed and two segfaulted in existing replay-gain/controller-screen tests:

• ControllerScriptEngineLegacyTest.screenWillSentRawDataIfConfigured
• AdjustReplayGainTest.AdjustReplayGainUpdatesPregain

The controller-screen test passes when isolated. The replay-gain test reproduces locally in the mixxx-build container, and A/B probing shows that registering an additional EngineControl exposes the failure; no safe root-cause fix was established. This remains a pre-existing validation issue for the broader feature branch.

Pre-existing local AGENTS.md changes and src/test/echoreverbchain_test.cpp were deliberately excluded.

[0cwa]

CodeRabbit disposition audit at PR23 head c93db5c0; the confirmed fixes are included in PR31 head 842644d.

Confirmed fixed in PR31: 3217, 3223, 3253, 3277.

All other reviewed non-outdated roots are stale or intentional:

• Stale — build/tooling, vendored/offline dependency behavior, and CI checks: 3181, 3190, 3195, 3200, 3201, 3204.
• Stale — translation/theme labeling and compile guards: 3209, 3210.
• Stale — audio recovery, guards/lifetime, and reconfiguration paths: 3214, 3230, 3234, 3235, 3240, 3248.
• Stale — state/history/cue and model verification: 3251, 3258, 3260, 3261, 3266, 3267, 3270, 3271, 3273, 3274, 3276.
• Intentional — feature gating/build policy: 3198; invalid-state and retry semantics: 3212, 3237; negative visual/preroll positions required by engine semantics: 3245; platform temporary-path behavior: 3269.