release(v4.6.0): broadcast channels — Signal sender-keys for one-to-many fan-out

Lands the broadcast-channel primitive Prism asked for in
Docs/shade-feature-request-sender-keys.md. The crypto in
@shade/core/sender-keys.ts was already in place; this release wires
it up as a first-class app-facing API, adds the persistence schema
across all six storage backends (memory, sqlite, indexeddb +
encrypted variants), introduces wire type 0x21 in @shade/proto,
and ships Prism's three acceptance tests verbatim.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shade-files/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@shade/files",
-  "version": "4.5.0",
+  "version": "4.6.0",
   "type": "module",
   "main": "src/index.ts",
   "types": "src/index.ts",