docs: fix checkout status lifecycle diagram to match spec prose

[SumuduLansakara]

Description

The Checkout Status Lifecycle diagram only showed a one-way path from
ready_for_complete into complete_in_progress, with no way back to
incomplete and no path to requires_escalation. The normative prose
elsewhere in this file describes both:

• Complete Checkout's response section states that any other status
  retains its core status lifecycle semantics, explicitly naming
  incomplete ("for a recoverable change") and requires_escalation
  ("for Buyer handoff") as valid outcomes alongside completed and
  complete_in_progress.
• ready_for_complete can regress to incomplete via Update Checkout
  as well (business rules can reintroduce a missing requirement).

Update the diagram to show:

• ready_for_complete -> requires_escalation, labeled with the
  Complete Checkout escalation path.
• ready_for_complete <-> incomplete as two directional arrows: the
  existing "all info collected" transition down, and a new "Update
  Checkout, or Complete Checkout (recoverable)" transition back up.

Add a note documenting the two transitions still omitted from the
diagram for readability (complete_in_progress resolving via Buyer
handoff or via the cancellation race described in Accepted
completion), so the diagram's simplifications are explicit rather
than silent.

No normative or schema changes; documentation only.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

None filed — found while reading the spec, opening directly as a docs fix.

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

Before:

       +------------+                         +---------------------+
       | incomplete |<----------------------->| requires_escalation |
       +-----+------+                         |   (buyer handoff    |
             |                                |  via continue_url)  |
             | all info collected             +----------+----------+
             v                                           |
    +------------------+                                 |
    |ready_for_complete|                                 |

After: see the updated diagram in docs/specification/checkout.md.

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