feat(testcentre): master "Diagnostics capture" switch to gate on-device research artifacts

[pipiche38]

Summary

Adds a single "Diagnostics capture" master switch to Test Centre → Diagnostic tools (default OFF, opt-in). Today the Oura Tier-B JSONL sidecars are always-on and fill /OpenWhoop/Diagnostics during normal use; this puts every on-device research artifact behind one switch a tester flips on for a capture session.

Motivation

The Diagnostics folder accumulates artifacts (oura-activity, and — on other branches — oura-ibihr / oura-raw, plus the WHOOP raw-frame capture) with no user control. A single opt-in switch keeps the folder empty in normal use and gives a clean way to arm capture only when debugging.

What changed

• Shared pref key noopDiagnosticsCapture — identical string on both platforms (Swift PuffinExperiment.diagnosticsCaptureKey / Kotlin PuffinExperiment.KEY_DIAGNOSTICS_CAPTURE), following the existing PuffinExperiment cross-platform idiom.
• Oura sidecars gated — OuraActivityDump is only created when the switch is on (both platforms), so default-off writes no sidecar file. Future oura-ibihr / oura-raw honor the same key when they merge.
• WHOOP raw-frame capture folded in — a new rawCaptureActive helper (per-strap enableRawCapture/isCaptureEnabled OR the master) drives the 24/7 capture gate, so the master switch turns raw capture on too while the independent per-strap flag keeps working. The on-demand bounded raw-capture window is untouched.
• Test Centre UI — a toggle in the Diagnostic tools card on both platforms (Swift @AppStorage, Kotlin Switch), with copy explaining it applies on the next connection.

Cross-platform parity

Symmetric Swift/Kotlin: same pref key, same gate placement (dump creation + raw-capture guard), same UI toggle. No stored-data or analytics change, so nothing crosses the .noopbak boundary.

Behavior change

The Oura activity/MET sidecar was always-on and is now OFF by default — testers must enable the switch before a capture night. This is the intended effect (stop the folder filling).

Scope / notes

• Takes effect on the next ring/strap connection (read where each dump/capture is created), not mid-session.
• No protocol change — the raw-capture edits only widen a read-only capture gate; no BLE write, no connection-path change.
• On this branch only oura-activity exists to gate; oura-ibihr / oura-raw live on the combined/anchor branches and plug into the same key on merge.

Verification

• App-target Swift (CI doesn't cover it): Strand macOS xcodebuild … build → BUILD SUCCEEDED.
• Android: compileFullDebugKotlin → clean.
• Design system: toggle uses StrandPalette/StrandFont (Swift) and Palette/NoopType (Android) — no hardcoded colors/fonts.
• 9 files, single commit 5e3fb4d.
• On-device: pending your confirmation that flipping it toggles the oura-activity sidecar on/off.