docs(durable): read only through tx inside a commit callback

Session, Harness, and runtime reads such as snapshot(), documentState(), watchDoc(), and getTask() may queue on the mutation line a commit callback holds; on a cache miss they never resolve. Documented on Session.commit, TaskRuntime.commit, and in the spec's pitfalls.

packages/durable/docs/spec.md

@@ -5004,6 +5004,11 @@ These are contracts, not invitations to add defensive machinery:
 - **Long transactions:** an async commit callback holds the Session mutation
   line. Never await models, tools, processes, network calls, humans, a nested
   Session commit, or a Session waiter inside it. Use methods on the current `Tx`.
+- **Reads inside a commit:** `snapshot()`, `documentState()`, `watchDoc()`,
+  `getTask()`, and the other Session, Harness, and runtime reads may queue on the
+  mutation line, for example to load a document not in memory. Inside a commit
+  callback, which holds that line, such a read never resolves, and the line stays
+  held. Only a cache hit returns, so the hang is intermittent. Read through `tx`.
 - **Explicit creation:** only typed `tx.doc()` creates an absent document. Snapshot,
   state, and watch lookup return `undefined` instead.
 - **Family initialization:** the first acquisition of an absent family address

packages/durable/src/types.ts

@@ -191,7 +191,7 @@ export interface TaskRuntime<I, S, R, H extends object> extends DocumentObserver
 	 * Commit on the Session line after rereading the task. Rejects when the task is terminal, the invocation ended, the
 	 * Harness is closing, or, in a run invocation, the task carries an abort mark. A returned state replaces the task's
 	 * state in the same commit; returning nothing leaves it unchanged. `tx.createTask()` defaults to the task's
-	 * conversation.
+	 * conversation. Inside the callback read only through `tx`: `snapshot()`, `documentState()`, `watchDoc()`, `getTask()`, and the other Session and runtime reads may queue on the line the callback holds, and then never resolve.
 	 */
 	commit(
 		change: (
@@ -948,7 +948,9 @@ export type SessionEnd = { readonly reason: "closed" } | { readonly reason: "fai
 
 /** Owner of one mutation line, its records, and its tracked documents. */
 export interface Session extends DocumentObserver {
-	/** Run one atomic transaction on the Session mutation line. */
+	/**
+	 * Run one atomic transaction on the Session mutation line. Inside the callback read only through `tx`: `snapshot()`, `documentState()`, `watchDoc()`, `getTask()`, and the other Session and runtime reads may queue on the line the callback holds, and then never resolve.
+	 */
 	commit<T>(change: (tx: Tx) => T | Promise<T>, context: Context): Promise<T>;
 	/** Seal admission, settle admitted commits, then close storage. */
 	close(context: Context): Promise<void>;