mizan-webview-transport + webview-channels: VSCode webview as Mizan frontend

Two new frontend packages let a VSCode webview consume a Mizan backend
through its postMessage channel — peer transports to `mizan-tauri-transport`
and the default `httpTransport()`.

- `@mizan/webview-transport` implements `MizanTransport` (call/fetch)
  over postMessage with correlation-id pairing. Drop-in for `configure({
  transport: webviewTransport() })`; codegen output and React adapter
  are unchanged.

- `@mizan/webview-channels` mirrors mizan-react's WebSocket-based
  ChannelConnection — RPC + subscribe over the same postMessage channel
  for long-running ops where short request/reply isn't enough.

Both expect an extension-host-side dispatcher that reads envelopes via
`webview.onDidReceiveMessage` and routes them through mizan-ts's
`handleMutationCall` / `handleContextFetch`. First consumer is the
holomorphic VSCode extension.

mizan-codegen: new `[source.script]` generic source. Spawns an arbitrary
command and reads stdout as KDL IR. Keeps mizan-codegen out of the
business of knowing every possible backend language while preserving
the "subprocess emits KDL" contract every other source already follows.
Holomorphic uses it to invoke `python -m holomorphic.emit_ir` against
the mizan_core registry.

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

protocol/mizan-codegen/src/config.rs

@@ -69,6 +69,16 @@ pub struct SourceConfig {
     /// usage (no Pydantic) just omits the sub-block.
     #[serde(default)]
     pub rust: Option<RustSource>,
+
+    /// `[source.script]` — generic source. Spawn an arbitrary command and
+    /// read its stdout as KDL IR. Use when none of the language-specific
+    /// sources fit — e.g. a Python module that walks `mizan_core.registry`
+    /// for a non-Django/non-FastAPI consumer, or a custom IR emitter.
+    /// Keeps mizan-codegen out of the business of knowing every possible
+    /// backend language while preserving the "subprocess emits KDL"
+    /// contract every other source already follows.
+    #[serde(default)]
+    pub script: Option<ScriptSource>,
 }
 
 
@@ -206,6 +216,35 @@ fn default_pydantic_derives() -> Vec<String> {
 }
 
 
+/// `[source.script]` — generic stdout-of-arbitrary-command source.
+///
+/// Spawns `command` with `args`, reads its stdout, and parses it as KDL
+/// Mizan IR. The same contract every other source follows; this one just
+/// doesn't bake in any language-specific assumptions.
+///
+/// Example:
+///
+/// ```toml
+/// [source.script]
+/// command = ["uv", "run", "python", "-m", "holomorphic.emit_ir"]
+/// ```
+#[derive(Debug, Deserialize)]
+pub struct ScriptSource {
+    /// Full command vector. First entry is the program; rest are argv.
+    /// Must be non-empty.
+    pub command: Vec<String>,
+
+    /// Working directory for the subprocess, relative to the codegen
+    /// config directory. Defaults to the config directory itself.
+    #[serde(default)]
+    pub cwd: Option<PathBuf>,
+
+    /// Environment overrides.
+    #[serde(default)]
+    pub env: BTreeMap<String, String>,
+}
+
+
 #[derive(Debug, Deserialize, Clone)]
 #[serde(untagged)]
 pub enum RustKernelSpec {