feat: add plugin API v3 contract

[kachofugetsu09]

Summary

• add static validation for module-level Plugin API v3 exports
• keep API v2 class validation during the dual-entry migration window
• report API version and entrypoint identity in the machine-readable result

Validation

• python -m pytest -q (7 passed)
• pyright akashic_plugin_contracts tests (0 errors)
• black --check and compileall passed
• contract passed against Citation 7527251b88c7530b20685f38b5dbab6107fc1f5b and Meme 3ca7e1415d50c05a7f475595b26032f3db9faae2

Recovery

• backup bundle: /mnt/data/coding/akashic-plugin/.backups/20260814-pre-plugin-contracts-v3-6aba4ca.bundle

[kachofugetsu09]

这份草案服务于 v2/v3 双入口迁移窗口，仍明确保留 v2 Plugin class 合同；当前 pure-v3 迁移已在 Core 的 fleet/static composition gate 中完成并删除 v2 ABI。继续合并会重新引入过期的双轨语义，因此作为 superseded 关闭，不合并。