feat(policy): add portable UI policy controls

[shailendra-nv]

Summary

Add a platform-neutral, deny-by-default ui policy section and compute-driver
capability negotiation. MXC process_container maps the policy to MXC UI
controls, while unsupported runtimes reject every explicit UI section before
provisioning and preserve existing behavior when the section is omitted.

Related Issue

Closes #3022

Changes

• Add portable allow_graphical_ui, directional clipboard, and
  allow_input_injection policy controls to the Rust and protobuf models.
• Add a compute-driver capability for complete UI-policy enforcement and expose
  it through gateway health output and the Go SDK.
• Reject every explicit UI object, including ui: {}, when a selected driver
  does not advertise support. Docker, Podman, Kubernetes, VM, non-Windows
  builds, older extensions, and MXC isolation_session therefore remain
  default-deny for the new policy surface.
• Map UI policy only for MXC process_container; keep omitted policy
  restrictive and reject unknown enum values or lossy mappings.
• Update policy hashing/static-update checks, schema documentation,
  architecture documentation, MXC documentation, policy-generation skills,
  Windows validation tasks, and Go SDK conversions.
• Add policy parsing/validation, capability, unsupported-driver, mapper,
  lifecycle, real-MXC schema, SDK, and end-to-end tests.

Testing

Validated head: 69ee7d145f9c4c0e0637f830a134700b5b96da70

• mise run pre-commit on native Windows x64
• windows:check:x64
• windows:build:x64
• windows:test:x64 — 3,569 passed, 18 skipped, 0 failed
• windows:test:unsupported:x64 — all driver/feature combinations passed
• windows:test:mxc-real:x64 — 11 passed, 0 failed; unavailable live
  backend facilities reported as explicit skips
• windows:e2e:mxc:mock — 4 passed, 0 failed
• Windows ARM64 check, build, lint, workspace tests, unsupported-driver
  tests, real-MXC schema tests, and mock E2E validation
• Unit tests added/updated
• E2E tests added/updated

The broader Windows test aggregate also completed 3,569 Rust tests, SBOM tests,
release-range tests, and documentation tests successfully. It then reached a
Unix bash parity task that does not apply on native Windows.

MXC compatibility:

• Latest released MXC 0.8.0+6cd3d58f passes the real-MXC suites.
• Microsoft MXC main at 3a22a2b94a81da081d260b087c4c7faeb7be7f14
  builds with and without isolation_session, and passes the real-MXC suite in
  both configurations.
• Representative mapper output validates against MXC stable 0.8.0-alpha, dev
  0.9.0-alpha, and rolling 0.9.0-dev schemas.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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[drew]

Reviewed commit de2f834. Found four issues, detailed inline: global-policy ingress bypasses UI enforcement checks, MXC accepts grants suppressed by its disable flag, YAML roundtrips change the default clipboard representation, and the Go fake client shares mutable UI state.

Validation: all 10 targeted existing Rust UI tests and the Go converter/fake suites pass. Additional regression checks reproduced global-policy acceptance and the clipboard roundtrip mismatch; a Go reproduction confirmed fake-client aliasing. The MXC finding is based on the mapper and Microsoft's documented enforcement contract; Windows enforcement was not executed on this Linux host. Required branch CI was still pending when reviewed.