v0.3.6 — the "to fill in" area now survives refreshes

v0.3.6 — a fidelity fix, not a feature

The auto-draft block is replaced between its begin/end markers. So: an agent fills in the
"to fill in" slots → occupancy climbs 5 points → the refresh wipes what it wrote. A wrong
conclusion can be corrected; a vanished one is not even noticed.

Fixed: the to-fill-in area now sits between <!-- dsh-context-guard:todo:begin/end -->. On refresh the
plugin re-emits that region byte-for-byte into the new block, while the machine facts above it are
updated. Blocks written by ≤0.3.5 have no sub-markers and are rescued once via a - [ ] line anchor
(language-independent, so zh and en blocks both work).

Docs: both READMEs gain a "How faithful is the handoff?" table — machine facts are verifiable,
the turn summary is truncated (a clue, not evidence), and the to-fill-in slots are empty because
the plugin has no idea what you did. Two consequences are spelled out: an agent that only sees the
summary will infer (read its conclusions as a draft), and the old session's agent writes better
conclusions because it still has the context. The banner button's tooltip now says it writes machine
facts only.

Tests: +2 (manual notes survive a refresh; a legacy block is rescued via the - [ ] anchor), and
composeHandoverBlock now asserts the sub-markers and their order. 54/54 unit tests, selftest 81/81,
client bundle byte-identical (70,200 bytes).

中文

v0.3.6 —— 一次保真修复，不是新功能

自动草稿块是按 begin/end 标记整块替换的。于是：agent 填好「待补」→ 水位再涨 5 个点 → 刷新
→ 刚写的内容被冲掉。补错还能改，丢了你还不知道。

已修：待补区现在包在 <!-- dsh-context-guard:todo:begin/end --> 之间；刷新时插件把这一段
原样搬进新块，只更新上面的机器事实。≤0.3.5 写的旧块没有子标记，靠 - [ ] 行锚点兼容抢救一次。

文档：两版 README 都补了「保真的边界」表 —— 机器事实可核对、轮次摘要是截断的（线索而非证据）、
待补是空的（插件不知道你干了什么）。并写明两条推论：只看到摘要的 agent 会推断（当草稿读）；
旧会话的 agent 写结论更准（它还有上下文）。按钮 tooltip 也改成「只写机器事实」。

测试：新增 2 项（刷新后人工内容仍在；旧块靠 - [ ] 锚点抢救），compose 另加子标记与顺序断言。
单测 54/54、自检 81/81、bundle 逐字节同源（70,200 字节）。