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/retcons.md

@@ -0,0 +1,12 @@
+# 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 |
+|---------|--------------|-----|
+|         |              |     |