fix: isolate user-created topics in regular groups

[hu5h]

Summary

• Give each user-created Lark topic in a regular group its own thread-scoped botmux session.
• Anchor a topic seed at its message_id, then reuse root_id for later replies.
• Keep explicit user-created topics isolated under regular-group shared reply mode.
• Preserve topic-group auto-start, DM chat mode, shared aliases, and rooted-thread behavior.

Problem

Regular groups normally use one chat-scoped session. When a user explicitly creates an independent Lark topic, its root event can contain thread_id=omt_* without root_id. That root previously fell through to the regular-group routing policy and reused the group session, so the topic did not have isolated context.

Implementation

• Classify thread-root events after chat-mode resolution so topic-chat seeds retain source=topic-chat, while only ordinary-group topic roots become real-thread sessions.
• Prevent shared mode from folding an explicit user-created topic root back into the group session.
• Add routing and full dispatcher regressions for root/reply continuity, existing chat sessions, shared mode, and topic-chat auto-start.

Validation

• pnpm vitest run test/event-dispatcher.test.ts — 121/121 passed
• pnpm exec tsc --noEmit — passed
• pnpm build — passed
• git diff --check — passed
• Independent read-only review — APPROVE
• Local Lark deployment and user acceptance test — passed

Source

Merged and validated first in hu5h#1 (6dbc20c7).

[hu5h]

Superseded by #631, which ports the same focused change onto the latest upstream master and is mergeable. Closing this conflicting fork-master PR to keep review on the clean branch.