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-storage-encrypted/src/crypto/row-codec.ts

@@ -39,6 +39,7 @@ export const COL = {
   trustedIdentity: 'trusted_identity',
   retiredIdentity: 'retired_identity',
   streamSensitive: 'stream_sensitive',
+  broadcastChannelSensitive: 'broadcast_channel_sensitive',
 } as const;
 
 /** Logical table identifiers — used for fieldKey + AAD binding. */
@@ -51,6 +52,7 @@ export const TBL = {
   trustedIdentities: 'trusted_identities',
   retiredIdentities: 'retired_identities',
   streamState: 'stream_state',
+  broadcastChannels: 'broadcast_channels',
 } as const;
 
 /** Encrypt an arbitrary string payload bound to (table, column, pk). */
@@ -226,3 +228,76 @@ export async function openStreamSensitive(
   if (b.overallHashState !== undefined) (out as { overallHashState?: string }).overallHashState = b.overallHashState;
   return out;
 }
+
+/**
+ * Broadcast-channel sensitive bundle (V4.6). Routing fields (channelId,
+ * ownerRole, ownerAddress, label, generation, createdAt, updatedAt) live
+ * in plaintext columns so backends can list/query without unsealing every
+ * row; the chain key, iteration, and signing keys all live in this sealed
+ * blob.
+ */
+interface BroadcastChannelSensitiveBundle {
+  chainKey: string;             // base64(32B)
+  iteration: number;
+  signingPublicKey: string;     // base64(32B)
+  signingPrivateKey?: string;   // base64; only when ownerRole === 'sender'
+}
+
+export async function sealBroadcastChannelSensitive(
+  km: KeyManager,
+  channelId: string,
+  s: {
+    chainKey: Uint8Array;
+    iteration: number;
+    signingPublicKey: Uint8Array;
+    signingPrivateKey?: Uint8Array;
+  },
+): Promise<Uint8Array> {
+  const bundle: BroadcastChannelSensitiveBundle = {
+    chainKey: toBase64(s.chainKey),
+    iteration: s.iteration,
+    signingPublicKey: toBase64(s.signingPublicKey),
+  };
+  if (s.signingPrivateKey !== undefined) {
+    bundle.signingPrivateKey = toBase64(s.signingPrivateKey);
+  }
+  return sealString(
+    km,
+    TBL.broadcastChannels,
+    COL.broadcastChannelSensitive,
+    channelId,
+    JSON.stringify(bundle),
+  );
+}
+
+export async function openBroadcastChannelSensitive(
+  km: KeyManager,
+  channelId: string,
+  blob: Uint8Array,
+): Promise<{
+  chainKey: Uint8Array;
+  iteration: number;
+  signingPublicKey: Uint8Array;
+  signingPrivateKey?: Uint8Array;
+}> {
+  const json = await openString(
+    km,
+    TBL.broadcastChannels,
+    COL.broadcastChannelSensitive,
+    channelId,
+    blob,
+  );
+  const b = JSON.parse(json) as BroadcastChannelSensitiveBundle;
+  const out: {
+    chainKey: Uint8Array;
+    iteration: number;
+    signingPublicKey: Uint8Array;
+    signingPrivateKey?: Uint8Array;
+  } = {
+    chainKey: fromBase64(b.chainKey),
+    iteration: b.iteration,
+    signingPublicKey: fromBase64(b.signingPublicKey),
+  };
+  if (b.signingPrivateKey !== undefined) out.signingPrivateKey = fromBase64(b.signingPrivateKey);
+  return out;
+}