Refuse a single unreadable PDU instead of the whole link (#79)

* Regression tests: a PDU the codec cannot read costs that PDU, not the link

* Refuse a single unreadable PDU instead of the whole link

* smscsim interop asserts the first attempt gets every DLR

* Record the smscsim sequence number defect as fixed

* One framing rule and one response-command lookup, per the architecture review

* Apply the stability review's nits: honest sessionError docs and one link-survives assertion

src/pdu-framer.ts

@@ -1,5 +1,5 @@
 import type { Result } from './result.ts';
-import { maxPduLength } from './pdu.ts';
+import { framingRefusal } from './pdu.ts';
 
 /**
  * Cuts a byte stream into whole PDUs.
@@ -31,10 +31,9 @@ export class PduFramer {
 
 		while (this.length >= 16) {
 			const cmdLength = this.join(16).readUInt32BE(0);
+			const unframable = framingRefusal(cmdLength);
 
-			if (cmdLength < 16 || cmdLength > maxPduLength) {
-				return { err: new Error(`Refusing a cmd_length of ${String(cmdLength)}`) };
-			}
+			if (unframable) return { err: unframable };
 
 			if (this.length < cmdLength) break;