[P0][OS-02] Servo and alternative-engine compatibility track

Programme: #1
Epic: #34

## Objective
Maintain a real Rust-native rendering-engine path behind the browser-engine abstraction without sacrificing website compatibility or security for language purity.

## Scope
- Engine contract for navigation, rendering surfaces, source/origin identity, observations, input/focus and feature capability reporting.
- Integrate Servo or alternative Rust engine into isolated test/application workloads.
- Compatibility corpus covering public pages, forms, authentication handoff, accessibility, CSS/layout, media and common application patterns.
- Security/process isolation assessment and update cadence.
- Performance/resource comparison against Chromium/CEF on source-backed workspace/browser scenarios.
- Per-origin/workload engine-selection experiment only after compatibility/safety evidence.
- Explicit Chromium fallback and user/support diagnostics.

## Product rules
- Alternative engine is enabled only where measured compatibility meets the declared workload gate.
- Original-view authenticity/functional requirements take precedence over Rust implementation purity.
- Engine differences remain visible to diagnostics/evaluation.

## Acceptance criteria
- [ ] Browser-engine abstraction supports Chromium and at least one experimental alternative without workspace/data contract changes.
- [ ] Compatibility results are reproducible against EVAL-01 corpus.
- [ ] Unsupported workloads automatically/usefully fall back to the supported engine rather than silently breaking.
- [ ] Accessibility, focus/input and origin identity are independently evaluated.
- [ ] Security/update ownership is defined before any production enablement.
- [ ] No product claim presents experimental engine coverage as universal web compatibility.

## Dependencies
- WEB-01
- WEB-04
- EVAL-01

**First phase:** P0
**Maturity target:** P8 / opportunistic
**Owner:** os-systems

Start by reading the linked WEB-01, WEB-04, and EVAL-01 dependencies to understand the browser-engine contract and compatibility corpus. Define the experimental alternative-engine path, reproducible compatibility evaluation, fallback behavior, accessibility and security checks, and ownership criteria required by the acceptance checklist.

Written by the indexing model from the issue text.