[P1][WEB-04] Original-view continuity and engine abstraction

Programme: #1
Epic: #15

## Objective
Make switching between personalized workspace UI and authentic service UI reliable, while keeping the rendering engine replaceable behind a compatibility contract.

## Scope
- Stable source/object deep links from Personal/Evidence views to Original view.
- Correct account/profile selection for handoff.
- Preserve safe navigation continuity: back/forward, workspace return point, relevant source location where supported.
- Bookmark/deep-link representation that can target workspace objects and original URLs.
- Browser-engine abstraction for navigation, observation, rendering surfaces and feature capability reporting.
- Explicit handoff metadata when state cannot be transferred between generated UI and original service.
- Servo/other engines plug into the same interface only for tested workloads.

## Correctness rules
- Never claim cart/form/session state was transferred unless verified.
- Account mismatch must block or visibly require the correct profile.
- Source links remain authentic URLs/IDs; generated UI never impersonates the source.

## Acceptance criteria
- [ ] A Personal-view entity can open the correct source object in the correct account context.
- [ ] Returning from Original view restores the workspace location and user layout state.
- [ ] Unsupported state transfer is explicitly disclosed rather than inferred.
- [ ] Engine capability differences are queryable and used for truthful fallback.
- [ ] Deep links survive application restart where source/account permissions still allow them.

## Dependencies
- WEB-01
- WS-01

**First phase:** P1
**Maturity target:** P5
**Owner:** platform

Review dependencies WEB-01 and WS-01 first, then map the Personal, Evidence, and Original views and the browser-engine compatibility boundary. Done means source links open in the correct account context, workspace state returns correctly, unsupported transfers are disclosed, and engine capabilities support truthful fallback.

Written by the indexing model from the issue text.