Refuse a body the PDU's own data_coding cannot carry (#98)

* Regression tests for a body the PDU's own data_coding cannot carry

* Refuse a body the PDU's own data_coding cannot carry, in the codec both send paths build through

* Record the codec's body refusal, and drop two todo entries it and the interop run close

* Regression tests for the aliased body TLV and the data_coding short_message settles

* Find the body TLV by tag id, leave data_coding to the field that will be read, and correct the record

* Record the two architecture findings this change does not close

* Regression tests for a duplicated body tag and a short_message the wire never carries

* Resolve every body TLV against the field that will be read, and move TLV writing to its table

* Regression tests for a body only the TLV carries and an empty short_message

* Let only octets the command writes settle the alphabet, and stop shadowing the TLV table

src/defs/encodings.ts

@@ -123,6 +123,13 @@ export function detect(value: string): EncodingName {
 
 export type Unencodable = { char: string; index: number };
 
+/** What an alphabet could not carry, said the one way: the character, its code point and its index. */
+export function unencodableText(at: Unencodable): string {
+	const point = (at.char.codePointAt(0) ?? 0).toString(16).toUpperCase().padStart(4, '0');
+
+	return `${JSON.stringify(at.char)} (U+${point}) at index ${String(at.index)}`;
+}
+
 /** The first character `encoding` cannot carry, or undefined where it carries every one of them. */
 export function unencodable(message: string, encoding: EncodingName): Unencodable | undefined {
 	const codec = encodings[encoding];