Document local narration installation and troubleshooting

## Problem

Users and maintainers need accurate guidance on optional installation, offline privacy, hardware limits and recovery, not assumptions from the exploratory PRD.

## Proposed change

Update existing user, architecture and desktop-bundle documentation with the delivered controls, supported hardware, measured sizes/resources, local-only privacy boundary, disable versus remove, deliberate updates, incompatible/corrupt installation recovery, runtime failures, and deterministic fallback. Document prompt/artifact versioning and how to rerun the evaluation/benchmark process. Add the appropriate Unreleased changelog entry; no speculative future features.

## Acceptance Criteria

- [ ] AC1: A documentation walkthrough against actual desktop/TUI/plain controls successfully follows install, enable, disable, remove and recovery instructions.
- [ ] AC2: A documentation-to-manifest/benchmark check confirms supported platforms, size/memory expectations, license attribution and privacy statements match the release inputs.
- [ ] AC3: A maintainer walkthrough identifies pinned artifact/prompt versions and reproduces the documented evaluation command without cloud credentials.
- [ ] AC4: The Unreleased changelog describes the delivered optional behavior and relevant limitations; verified by release documentation review.

## Implementation classification

- **Estimated size:** S
- **Orchestration risk:** Normal
- **Reason:** This documents the established feature rather than introducing another behavior.

## Dependencies

Direct prerequisites: #461, #462, #463, #464, #465. Consumer: Verify the complete offline narration release journey.