[cbaucom]

Closes #13

Approach

Pure-module Beat Grid implementation in src/beat-grid/ with planner integration.

Detection (detection.ts): energy-based onset detection (RMS per 512-sample hop → positive first-difference) followed by autocorrelation over the 60–200 BPM lag range. Phase accumulation then finds the first-beat offset. All math is parameter-free and tested via synthetic click tracks.

Nudge (nudge.ts): snaps a slide's target duration to the nearest beat multiple. Energy applies a multiplier before snapping (calm 1.5×, medium 1.0×, punchy 0.67×) so average durations differ by energy level while energy=medium always stays within ±½ beat interval of the passed-in target.

Planner wiring (planner.ts): new optional beatGrid? parameter; beatSyncActive = !!beatGrid && settings.beatSync !== false; getDuration calls nudge for non-title, non-video slides. Energy is read via resolved(slide).energy so per-slide overrides are honoured correctly.

Schema (schema.ts): beatGridCache?: BeatGrid added to SlideshowJson for future persistence.

Adapter (adapter.ts): thin decodeMono wrapper over AudioContext.decodeAudioData for use in the next UI-integration slice.

Acceptance criteria

• Given a synthesized click track of known BPM, when analyzed, detected BPM is within ±2% and offset within ±30ms — tested in detection.test.ts at 90 and 120 BPM
• Given beat sync on, when planned, every slide boundary lands on a beat and each duration deviates from its target by at most half a beat interval — tested in planner.test.ts "beat sync on (AC2)"
• Given beat sync off, when planned, durations equal targets exactly — tested in planner.test.ts "beat sync off (AC3)"
• Given each energy level on the same timeline+track, when planned, average slide duration differs in the expected direction — tested in both nudge.test.ts and planner.test.ts "beat sync energy effect (AC4)"

Code review verdict

APPROVE — one confirmed finding fixed: energy now reads through resolved(slide) instead of settings directly, so per-slide energy overrides in SlideOverrides are honoured. Also added Math.max(lagMin, ...) guard on lagMax for non-standard sample rates.

Follow-ups noted (out of scope)

• Wiring decodeMono + detectBeatGrid into the editor shell (UI integration slice)
• Persisting beatGridCache through slidesToJson / read-back in Project Store
• UI controls for beatSync toggle and energy selector in GlobalSettingsPanel