feat: add the-ballad-of-chalk campaign

Cloud Empress campaign for 弗莱格, a sellsword obsessed with chalk.
Ships the brief, PC sheet, state files, a starting location, and the
session 0001 log. The opening hook and hidden truth live in gm/.

campaigns/the-ballad-of-chalk/gm/README.md

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+# GM only
+
+Truth the PC has not learned: hidden motives, unrevealed identities, off-screen
+events, and what is really going on.
+
+**Not player-facing.** The player may read anything else in the campaign, so a
+secret must never be written outside `gm/`. Nothing here may appear in a session
+log or in chat until the PC observes it or is told.
+
+Keep `../state/knowledge.md` as the PC's counterpart: what they know, believe,
+and suspect. When the two disagree, `knowledge.md` is what the PC currently
+thinks — narrate from it, not from here.
+
+## Retconning
+
+You may change a fact here after the fact, but only in the layer the PC has not
+perceived. The test is perception: if every session-log entry and everything in
+`state/knowledge.md` still reads exactly as before, the retcon is invisible and
+allowed. What the PC saw, heard, or was told stays fixed; the cause behind it
+may change.
+
+Use it to repair a plot that cannot otherwise be made to work. Do not use it
+liberally, and never to dodge a consequence the fiction already earned. Record
+each one in `retcons.md`.

campaigns/the-ballad-of-chalk/gm/notes.md

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+# 幕后真相（GM only）
+
+**不可出现在会话日志或对话中**，直到弗莱格亲眼看见或被人告知。
+
+## 泥巴孩子
+
+- 神谕：`Q: 孩子的村庄是被巨蝉毁灭的吗？ d6=3 → No, but…`
+  村庄**不是**毁于巨蝉。真实原因**尚未确立**——留给神谕在合适时机决定。
+- 同一次神谕的转折：`某个同行者一直在撒谎，你此刻才看见破绽。`
+  有人在撒谎，而破绽此刻已经在场。
+- 已确定的破绽：孩子浑身是**湿泥**，但这一带是盐碱荒原，最近的水源是灰喉井，
+  方向也对不上。孩子自称从荒地深处走来——说不通。
+- 「我是村庄唯一的幸存者」这句话可能为假。
+- 撒谎者未必要是孩子本人；也可能是弗莱格随后遇到的某个同行者。但破绽现在就在场。
+
+## 白垩
+
+- 弗莱格对白垩的执念是他自己的事，**不是**世界秘密。
+- 他的父母是否真的需要「被证明」、他们现在何处、是否还活着——**尚未确立**。
+  用 `journal oracle` 决定，不要凭空编。
+
+## 待定（用神谕或火花表确立，别硬编）
+
+- 村庄真正的遭遇。
+- 孩子是否知道真相，以及她在隐瞒什么。
+- 白垩坡这个地名是否真实存在。

campaigns/the-ballad-of-chalk/gm/retcons.md

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+# Retcons
+
+A log of facts changed after the fact. Each entry records what changed and why,
+so the change is not quietly contradicted later.
+
+A retcon is only allowed in the layer the PC has not perceived: every session
+log entry and every line of `../state/knowledge.md` must still read exactly as
+before. See `README.md`.
+
+| Session | What changed | Why |
+|---------|--------------|-----|
+|         |              |     |