feat(durable-streams): add stat method and StatResult to DurableRuntime interface

Adds a stat(path) method that returns file metadata without reading
contents. Returns { exists: false, isFile: false, isDirectory: false }
for missing paths instead of throwing — existence checks should not
require try/catch. Permission errors still propagate.

Author: Taras Mankovski

durable-streams/mod.ts

@@ -64,6 +64,7 @@ export type {
   DurableRuntime,
   ResponseHeaders,
   RuntimeFetchResponse,
+  StatResult,
 } from "./runtime.ts";
 
 // Context

durable-streams/runtime.ts

@@ -38,6 +38,18 @@ export interface RuntimeFetchResponse {
   text(): Operation<string>;
 }
 
+/**
+ * Result of a `stat` call.
+ *
+ * For missing paths `stat` returns `{ exists: false, isFile: false, isDirectory: false }`
+ * instead of throwing — "does this exist?" has "no" as a valid answer.
+ */
+export interface StatResult {
+  exists: boolean;
+  isFile: boolean;
+  isDirectory: boolean;
+}
+
 /**
  * Platform-agnostic runtime for durable effects.
  *
@@ -56,6 +68,15 @@ export interface DurableRuntime {
   /** Read a text file. */
   readTextFile(path: string): Operation<string>;
 
+  /**
+   * Check file/directory existence and type. Never throws for missing paths.
+   *
+   * Returns `{ exists: false, isFile: false, isDirectory: false }` when the
+   * path does not exist. Permission errors and other filesystem errors still
+   * throw — they indicate a real problem, not "file doesn't exist."
+   */
+  stat(path: string): Operation<StatResult>;
+
   /** Expand glob patterns. Returns relative paths with isFile flag. */
   glob(options: {
     patterns: string[];