Add archive key id support

docs/client-integration.md

@@ -40,6 +40,7 @@ Important:
 
 - the generated client header no longer embeds the real master key
 - `.m2p` loading now requires a runtime master key
+- the runtime master key must match the archive `key_id`
 - if a `.m2p` file exists and fails validation or runtime key resolution, the client should not silently fall back to `.pck`
 
 ## Runtime validation

docs/mcp.md

@@ -43,6 +43,7 @@ Inputs:
 - `output_archive`
 - `key_file`
 - `signing_secret_key_file`
+- `key_id` optional
 
 ### `pack_diff`
 
@@ -80,6 +81,7 @@ Inputs:
 - `key_file`
 - `public_key_file`
 - `output_header`
+- `key_id` optional
 
 ### `pack_binary_info`