markMeChatRead is chat-granular and over-claims read state when the timeline is truncated or filtered

[liuchao-001]

packages/web/src/pages/workspace/center/chat-by-id.tsx fires markMeChatRead(chatId) unconditionally on mount, on every chat:message frame for the open chat, and on ws:reconnect. The server-side mark is chat-granular — it has no watermark concept — so it clears the conversation list's unread badge / mention count for messages the viewer never actually saw:

• Pre-existing: while a blocking question truncates the timeline (takeover), newer messages behind the block are never rendered, yet an incoming chat:message still marks the chat read.
• Widened by feat(web): add timeline message filter and hand earlier-chat off to it #2164: with a persisted pair filter, every visit can be in a "has hidden messages" state; a hidden arrival's chat:message frame clears the list badge even though the row never rendered.

The local timeline anchors (durable watermark, persisted scroll anchor, live session high-water, divider anchor) are all bounded by the rendered-prefix ceiling since #2164, so in-chat attention (divider/pill) stays honest — the gap is only the server-side chat-level mark feeding the conversation list.

Fix directions both have costs and deserve a product decision:

1. Skip/defer markMeChatRead while the rendered timeline hides messages (list keeps showing unread for a chat the user has open — possibly correct, possibly annoying);
2. Teach the server mark a watermark (schema/API change);
3. Accept as-is and keep the boundary documented (current state — the Context Tree rule in first-tree-context#888 is explicitly scoped to local timeline anchors and names this exclusion).

Surfaced by the review agent during #2164 review (non-blocking, since the coarse behavior predates that PR).