feat(sidecar): protocol 14 — the Dangerous Treasures start forward (PLAN_REDESIGNS §14.3.4)

POST /dt/start, stamped dt.start whatever its body names, forwarded to the
plugin and answered with its dt.opened or dt.error. The plugin remembers the
key; DT despawns its own chest; nothing is kept here. dt.started, dt.won and
dt.ended ride the feed like any other event (docs rust-link/PROTOCOL.md §20.8).

72 tests (one new), fmt and clippy clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

sidecar/src/main.rs

@@ -198,6 +198,11 @@ use tracing_subscriber::EnvFilter;
 /// frames (24 of them, told apart by `event`) ride the feed like any other event, and hello's
 /// `console` like any other field.
 ///
+/// Dangerous Treasures (§20.8, the module's PLAN_REDESIGNS.md §14.3.4) adds one forward,
+/// `POST /dt/start`: open one treasure event for a run's step. The plugin remembers the key so a
+/// retry never opens a second; DT despawns its own chest. `dt.started`, `dt.won` and `dt.ended`
+/// ride the feed like any other event.
+///
 /// `docs/rust-link/PROTOCOL.md` is the specification — §8 the read path, §9 identity, §10 the
 /// mirror, §11 configuration, §12 clans, §13 the raid frame, §14 the leases, §15 the world verbs,
 /// §16 the rewards, §17 the map, §18 the optional mods, §19 the walk's fixes, §20 the step editor;