docs: close three retryability gaps in the Task 2 plan snippet

isRetryableStatus used an unbounded status >= 500, so a garbage status like
1000 landed in the retryable bucket. This function is the sole gate the
client and HTTP transport trust for retry safety, and an out-of-range value
must fail closed. Bounded on both sides.

408 Request Timeout was non-retryable while the same file documented
transport as covering "connection, timeout, 5xx" - a genuine timeout
surfaced as 408 was classified differently from the identical timeout
surfaced as 504. Now retryable.

There was no code for "the callee answered but not with a ServiceResult" - a
proxy's HTML error page, a truncated body. Task 8 was already papering over
it by hand-setting retryable: false beside a transport code that
retryableFor says is always retryable, which is exactly how the two drift
apart. Added RPC_MALFORMED and made that path use failure() so retryability
is derived from the code rather than written next to it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/plans/2026-08-05-inter-app-comms-implementation.md

@@ -285,16 +285,34 @@ describe("rpc errors", () => {
   });
 
   test("only transport failures are retryable", () => {
-    // 5xx and 429 are the callee saying "try again"; everything else is final.
+    // 5xx, 429 and 408 are the callee saying "try again"; everything else is final.
     expect(isRetryableStatus(500)).toBe(true);
     expect(isRetryableStatus(503)).toBe(true);
+    expect(isRetryableStatus(599)).toBe(true);
     expect(isRetryableStatus(429)).toBe(true);
+    expect(isRetryableStatus(408)).toBe(true);
     expect(isRetryableStatus(400)).toBe(false);
     expect(isRetryableStatus(403)).toBe(false);
     expect(isRetryableStatus(404)).toBe(false);
+    expect(isRetryableStatus(409)).toBe(false);
     expect(isRetryableStatus(200)).toBe(false);
   });
 
+  test("an out-of-range status fails closed rather than landing in the retry bucket", () => {
+    for (const status of [600, 1000, 0, -1, Number.NaN]) {
+      expect(isRetryableStatus(status)).toBe(false);
+    }
+  });
+
+  test("a malformed callee response is its own non-retryable code", () => {
+    const f = failure(RPC_ERROR_CODES.malformed, "Malformed service response");
+    if (!f.ok) {
+      expect(f.code).toBe("RPC_MALFORMED");
+      // Something answered; asking again returns the same thing.
+      expect(f.retryable).toBe(false);
+    }
+  });
+
   test("a denial is never retryable", () => {
     const f = failure(RPC_ERROR_CODES.denied, "Forbidden");
     if (!f.ok) expect(f.retryable).toBe(false);
@@ -344,6 +362,12 @@ export const RPC_ERROR_CODES = {
   handler: "RPC_HANDLER",
   /** Identity token missing, malformed, expired, or for another audience. */
   identity: "RPC_IDENTITY",
+  /**
+   * The callee answered, but not with a ServiceResult — a proxy's HTML error
+   * page, a truncated body, an unexpected shape. Distinct from `transport`:
+   * something DID respond, so retrying returns the same thing.
+   */
+  malformed: "RPC_MALFORMED",
 } as const;
 
 export type RpcErrorCode = (typeof RPC_ERROR_CODES)[keyof typeof RPC_ERROR_CODES];
@@ -354,11 +378,17 @@ function retryableFor(code: string): boolean {
 }
 
 /**
- * 5xx and 429 mean "the callee could not answer, try later". A 4xx is the
- * callee saying no — retrying it just repeats the same rejection.
+ * 5xx, 429 and 408 mean "the callee could not answer, try later". Any other
+ * 4xx is the callee saying no — retrying just repeats the same rejection.
+ *
+ * The range is bounded on BOTH sides deliberately: an unbounded `>= 500`
+ * puts a garbage status like 1000 in the retryable bucket, and this function
+ * is the sole gate the client and HTTP transport trust for retry safety.
+ * An out-of-range value must fail closed, i.e. not retryable.
  */
 export function isRetryableStatus(status: number): boolean {
-  return status >= 500 || status === 429;
+  if (status === 408 || status === 429) return true;
+  return status >= 500 && status <= 599;
 }
 
 export function success<T>(value: T): ServiceResult<T> {
@@ -1772,6 +1802,8 @@ export function httpTransport(options: HttpTransportOptions = {}): Transport {
       }
 
       if (!response.ok) {
+        // The one place retryability is status-derived rather than code-derived:
+        // the callee answered, and its status says whether asking again helps.
         return {
           ok: false,
           code: RPC_ERROR_CODES.transport,
@@ -1784,13 +1816,9 @@ export function httpTransport(options: HttpTransportOptions = {}): Transport {
         return (await response.json()) as ServiceResult;
       } catch {
         // A 200 that is not a ServiceResult means something else answered —
-        // a proxy, an error page. Retrying will not change that.
-        return {
-          ok: false,
-          code: RPC_ERROR_CODES.transport,
-          message: "Malformed service response",
-          retryable: false,
-        };
+        // a proxy, an error page. Retrying will not change that. Use failure()
+        // so retryability is DERIVED from the code, never hand-set beside it.
+        return failure(RPC_ERROR_CODES.malformed, "Malformed service response");
       }
     },
   };