feat(bridge): Backpacks on protocol 14 — read a backpack (PLAN_REDESIGNS §11.3)

WhiteThunder's Backpacks as an optional integration (D199), as §11.3.2 designs it.
Read, never write (R2):

- hello/status `integrations.backpacks` adds `sizes`: the N of every registered
  backpacks.size.N, ascending (dynamic-size profiles left out). Hello is resent
  when Backpacks loads or unloads.
- `backpacks.read {steamId}`: API_ReadBackpackContentsAsJson, and only when that
  answered a backpack, API_GetBackpackCapacityById — the backpack is loaded by
  then, so the capacity follows a later grant (§11.3.1's trap). Each stack is
  reduced to slot, shortname, display (the game's English name, D340), amount,
  skin and name when set; a blueprint carries its target as `blueprint` (D341);
  contents one level deep; `overflow` past the capacity. `pageSize` is
  Backpacks' "Max size per page" from its config, cached by mtime.
- Refusals: backpacks-missing, bad-id, unreadable.

The bigger backpack an event lends is a backpacks.size.N permission pushed by
perm.sync (D337), so it needs no code here. Still protocol 14 (D332).

Compiled on both rigs (Rust changeset 167584; Oxide 2.0.7826, Carbon 2.0.262),
then restored to the bridge they ran before.

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

README.md

@@ -105,6 +105,11 @@ Wulf's `Economics` is optional too, and not in `overlay.toml`. Where it is loade
 coins from an event, show balances and a richest column, and let staff adjust a balance; where it is
 not, each of those says "Economics is not installed on this server".
 
+WhiteThunder's `Backpacks` is optional in the same way. Where it is loaded staff can open a player's
+backpack from the site, a player can see their own, and an event can lend a bigger one for its run;
+the bridge only ever reads a backpack, and the bigger size is one of Backpacks' own `backpacks.size.N`
+permissions, pushed like any other grant.
+
 ## Diagnosing it
 
 ```