docs(demo): runnable demo of the pagination variant design (mindmap #16/#65)

[allmonday]

Summary

A runnable design document: the greenfield pagination design from the full-project orthogonality review (mindmap map 37, node #16 "分不分页 4 种编码" + its variant-design child node) landed as a self-contained demo script — no production code is touched.

uv run python demo/pagination_variant_design.py

What it demonstrates

Scenario	Point
1. Four relationships × five consumer points	The ideal design's daily life: resolver dispatch / SDL shape / introspect serialization / voyager icon — each consumer in one or two lines, zero truth-table knowledge
2. Side-by-side with the CURRENT production discriminator	Real RelationshipInfo instances run through the real is_active_paginated_relationship — 4/4 verdicts agree, proving the variant design changes structure, not behavior
3. The fate of an inconsistent state	Current: kind=COALESCED with a page_loader constructs silently; variant: TypeError — illegal states are unrepresentable, no validation needed
4. Capability vs assembly vs the local toggle	Your domain insight, live: local can_paginate=True always (SQL can slice anytime), execute_paginated flips with the toggle; remote paged ignores the toggle (capability = execution shape)

Three hands-on experiment points at the end of the file (remove paged_view_loader, flip Coalesced.paginated, add a new consumer point and count the lines).

Notes

• Self-contained: dataclasses + print only; imports production types solely for the scenario-2 comparison
• Pairs with the mindmap node (call_use_case 参数传入 JSON 字符串易出错，建议增强校验或提供示例工具 #16/security: arbitrary module import and source code disclosure in Voyager endpoints #65) and the eventual convergence PR (first stage: single discriminator function) — this file is the durable, runnable record of the target design

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