docs: explain loader mode ownership

[imbajin]

中文说明 (Chinese Summary)

背景与目标

在数据导入过程中，为防止多个 Loader 进程并发导入时相互覆盖图模式（Mode）状态，HugeGraph 引入了 LOADING 模式所有权 (Ownership) 机制。本 PR 同步完善 RESTful API 和 HugeGraph-Loader 的双语使用文档。

主要改动

1. RESTful API 参考 (graphs.md):
   • 新增 6.3.5 LOADING 所有权 小节：说明 /graphspaces/{space}/graphs/{graph}/mode/owners 接口的申请（acquire）与释放（release）协议。
   • 明确状态约束：当存在活跃的 Loader 所有者时，传统的 PUT /mode 枚举修改接口会被拒绝。
2. HugeGraph-Loader 指南 (hugegraph-loader.md):
   • 新增 3.4.6 并发导入与 LOADING 所有权 小节。
   • 平滑降级机制：升级后的 Loader 仅在探测返回路由级 404（即对接未支持该接口的旧版 Server）时，才会自动降级为旧版枚举模式；其余错误（鉴权失败、网络异常等）均直接报错退出，避免脏状态。
   • 异常恢复操作：补充未正常退出（如 kill -9）或句柄丢失时的手动 REST 释放及 Server 重启恢复指引。

边界与注意事项

• 保护范围：仅保护直连同一 Server 实例的普通 Loader 进程，防止模式被意外重置。
• 不支持场景：不提供跨 Server 的分布式锁协调，不提供数据写入级的事务隔离，所有权不设自动租约过期时间。
• 不受影响流程：旧版 Server、图数据源导入以及恢复导入（Restore）保持原有枚举模式行为。

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English Description

Summary

Update bilingual documentation for Loader concurrent import protection and LOADING mode ownership:

• Document the new /mode/owners RESTful APIs and handle lifecycle.
• Clarify Loader fallback boundaries and operational recovery steps.

Key Changes

1. RESTful API Reference (graphs.md):
   • Added 6.3.5 LOADING ownership: documents GET/PUT /graphspaces/{space}/graphs/{graph}/mode/owners (acquire / release actions).
   • Clarified mode restrictions: legacy PUT /mode is rejected while active owners hold the graph.
2. HugeGraph-Loader Guide (hugegraph-loader.md):
   • Added 3.4.6 Concurrent imports and LOADING ownership.
   • Fallback behavior: upgraded Loader only falls back to legacy enum mode on a route-level 404 (older Server); other errors (network, auth, acquire fail) fail immediately.
   • Recovery: documented manual REST release retries and handling of ungraceful exits (kill -9).

Scope & Boundaries

• Scope: protects ordinary Loader processes connected to a single Server instance.
• Not included: does not provide cross-Server distributed coordination, data write isolation, or automatic owner lease expiry.
• Unchanged flows: older Servers, graph-source, and restore imports keep their legacy behavior.

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Related Implementation PRs

• Server: apache/hugegraph#3331 (loading owner coordination)
• Toolchain: apache/hugegraph-toolchain#815 (Loader integration)
• Note: Hubble task cleanup is tracked separately in apache/hugegraph-toolchain#814.

Verification

• Hugo 0.165 Extended bilingual build passed
• Markdown links & anchors validated (dist/validate-links.sh)

[imbajin]

Independent current-head documentation review:8/10. REST owner semantics match the proposed Server; the Loader build-availability qualification remains open.

[bitflicker64]

Blocking: no. Summary: The REST and Loader text matches the paired implementations at their current heads, and the EN and CN pages agree; the inline comments cover four small gaps. The pages describe Server #3331 and Client/Loader #815 as unmerged, so please merge this after those two and then replace the wording about current master with the release that carries the feature. Evidence: read the full diff at 4ff825c (4 files, +155/-0); compared it with LoadingMode.java, GraphsAPI.java, HugeGraphAuthProxy.java and StandardHugeGraph.java in apache/hugegraph#3331 at 4a42a77, and with LoadContext.java, HugeGraphLoader.java and LoadingModeOwner.java in apache/hugegraph-toolchain#815 at 63cfd76; checked the mode setter roles on apache/hugegraph master 0b7c0de3e; checked that sections 6.3.5 and 3.4.6 follow the existing numbering and that both #loading-ownership anchors exist for the two cross-page links. No Hugo build was run locally; the Build group 0 check was still queued.