feat(m2): add ambient sounds with ocean waves

[Simon-byte-png]

What changed

• add a compact ambient-sound menu to the Hush Door
• provide white, pink, and brown noise plus ocean-wave and light-rain loops
• make ocean waves the default selection for new installs while keeping playback off until the user explicitly starts it
• persist the selected sound and volume locally
• use the iOS ambient audio session so playback mixes with existing audio without adding a background-audio entitlement
• document every public-domain source and SHA-256 checksum
• add a P4-to-P1 handoff for Xcode resource target membership

Why

The always-on companion needs an optional calm audio layer without adding network dependence, credentials, background playback, or extra instructional text to the minimal Hush Door UI.

Validation

• rebased onto the latest main
• HushMac builds successfully with code signing disabled
• all five MP3 files load and prepare successfully with AVAudioPlayer
• git diff --check passes
• no project.pbxproj, entitlement, Contract, networking, or backend changes

P1 follow-up

P1 still needs to add the five existing MP3 files to the Hush and HushMac targets under Copy Bundle Resources. The exact paths and exclusions are documented in docs/demo-ambient-sounds-handoff.md. This PR intentionally does not edit project.pbxproj because that file is owned by P1.