docs: fix the fullfilment discount examples and add totals example validator

[alexpark20]

Description

Two totals[] examples represent one free-shipping discount twice, so their declared total is short by the discount amount. This corrects both, scopes an invariant that was stated unconditionally, and adds a check so the arithmetic cannot drift again.

The misleading examples.

Both examples zero the fulfillment entry and emit an offsetting discount entry for the same free-shipping benefit, subtracting it twice:

• docs/specification/discount.md → "Mixed discounts (item + order level)": the line items net to 3200 and shipping is free, so the total is 3200. The example declared 2601.
• docs/specification/loyalty.md → "Price-Impacting Benefits": same shape, declared 651 where the line items net to 850.

Changes

1. Both examples corrected — the full shipping charge plus the offsetting discount, and the right total. Each discount entry now uses its own title as display_text so the pairing is legible.

2. The allocations invariant scoped to when allocations is present. It read as unconditional, yet allocations is optional on applied_discount and the free-shipping discount in both examples omits it entirely.

3. scripts/validate_totals.py enforces the two arithmetic identities the spec states normatively: a breakdown's entries sum to its total entry, and an entry's lines sum to that entry. Wired into pre-commit and the lint CI job.

Category (Required)

Please select one or more categories that apply to this change.

• 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

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)