[P0][PC-03] macOS and Linux native-control adapters

Programme: #1
Epic: #23

## Objective
Implement macOS and Linux backends for the common DesktopSession contract while respecting each platform's real accessibility, consent and input-control model.

## Scope
- macOS AXUIElement-based semantic observation/action adapter and accessibility permission lifecycle.
- Linux accessibility integration plus user-authorized desktop/input portals where appropriate.
- Window/application identity, semantic trees, text/value/action patterns and focus state.
- Platform-specific consent/permission UX and revocation detection.
- Bounded visual fallback using the same PC-01 freshness/postcondition rules.
- Reference application/workflow corpus per OS/display stack.
- Structured capability matrix for operations that differ by platform/session type.

## Design rules
- A remote-desktop/input portal is not treated as a semantic accessibility API.
- Permission grants are never self-approved by the general agent.
- Advertised capabilities are platform-specific and tested independently.

## Acceptance criteria
- [ ] Declared macOS workflows pass using AX semantics where available.
- [ ] Declared Linux workflows pass on supported desktop/session configurations.
- [ ] Permission denial/revocation fails visibly and cannot be bypassed by the agent.
- [ ] Input portal access is not misrepresented as semantic target verification.
- [ ] Freshness, user takeover and postcondition behavior matches PC-01 contract.
- [ ] Unsupported desktop environments/operations are visible in the support matrix.

## Dependencies
- PC-01

**First phase:** P0
**Maturity target:** P5
**Owner:** harness-pc-providers

Start by reading the PC-01 dependency and Epic #23 to understand the shared DesktopSession contract and programme context. Map the macOS and Linux accessibility, permission, portal, workflow, and display-stack requirements against the acceptance criteria; done means supported configurations are independently tested and unsupported operations are visible in the support matrix.

Written by the indexing model from the issue text.