feat(sound): add SoundService and SettingsView

[dinesh-git17]

Summary

Implements E8.1: Sound Service and Settings Toggle. Adds centralized AVAudioPlayer-backed sound playback service with named cue APIs for page turn, signature placed, and new content chime. Global sound preference persisted in UserDefaults (default: enabled). SettingsView provides deterministic toggle bound to SoundService state.

Design Doc Alignment

• S6.8: Three sound cues (page turn, signature placed, new content chime) with toggleable settings gear, default on.
• S10.5: AppState/service environment injection pattern.

Changes

• ExhibitA/ExhibitA/Core/SoundService.swift (created): @Observable final class with Cue enum, isSoundEnabled persisted preference, play(_:) API, AVAudioPlayer preparation with graceful missing-asset handling.
• ExhibitA/ExhibitA/Features/Home/SettingsView.swift (created): Settings surface with sound toggle using @Bindable binding to SoundService. Optional environment injection for preview safety. Theme-tokenized styling.
• ExhibitA/ExhibitA/App/ExhibitAApp.swift (updated): SoundService instantiation and .environment() injection.
• ExhibitA/ExhibitA.xcodeproj/project.pbxproj (updated): File references and build phase entries for both new files.

Risk Surface

• Audio assets (page_turn.caf, signature_placed.caf, new_content_chime.caf) are not yet in the bundle. SoundService handles this gracefully via Logger.info and no-op playback. Assets must be added before cues are audible.
• No call-site wiring in this PR. E8.2 wires SoundService.play(_:) into contract, signature, and sync surfaces.

Testing

• Built with xcodebuild -project ExhibitA.xcodeproj -scheme ExhibitA -sdk iphoneos build. Zero errors, zero code warnings.
• Protocol Zero scan: PASS (./scripts/protocol-zero.sh exit 0).
• Typographic lint: PASS (./scripts/check-em-dashes.sh exit 0).
• All pre-commit hooks pass.
• Device: verified build targets iphoneos (physical device "Din").
• Previews: SettingsView includes light, dark, and no-service previews.

Protocol Zero

./scripts/protocol-zero.sh exits 0.

Typographic Lint

./scripts/check-em-dashes.sh exits 0.

[github-actions]

CI Health Report

Workflow	Status	Notes	Additional Context
Content Validation	✅ success	--	0m 14s
Full Stack Smoke	✅ success	--	0m 31s
PR Governance	✅ success	--	0m 19s
iOS Build & Test	✅ success	--	0m 7s

Not triggered: API Sync Contract, Admin Smoke, Backend Quality, DB Schema Check, Dependabot Updates, Push Mock Integration, Release Readiness, iOS Assets & Theme Check, pages-build-deployment

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_{4 passed | 0 failed | 0 other | Commit: a389be7 | 2026-03-05 03:35:48 UTC}