feat(sidecar): protocol 14 — the Backpacks forward (PLAN_REDESIGNS §11.3)

POST /backpacks/read, a thin forward like /kits/uses: the body is stamped
`backpacks.read` whatever it names, so the route can only ever read. Nothing is
kept here. The bigger backpack an event lends is a permission in an ordinary
perm.sync (D337) and needs nothing new. Still protocol 14 (D332); one test.

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

sidecar/src/main.rs

@@ -188,6 +188,10 @@ use tracing_subscriber::EnvFilter;
 /// and `POST /economics/adjust` (staff). The plugin remembers a payment's key; this process keeps
 /// nothing. The tally's `balance` and the `economics.wiped` event ride the feed like any frame.
 ///
+/// Backpacks (§20.4, the module's PLAN_REDESIGNS.md §11.3) adds one: `POST /backpacks/read`,
+/// one player's backpack, read live. The plugin never writes a backpack, and a bigger one an event
+/// lends is a `backpacks.size.N` permission in an ordinary `perm.sync`, so it needs nothing here.
+///
 /// `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;