pipeline's stage ladder throws Node's body clause instead of crashing the compiler

- lowerStreamArg's arg-type marker escaped both pipeline lowerings uncaught (only finished caught it), so a compile-time non-stream stage — pipeline('', dst, cb) — crashed the CLI with a raw StreamArgTypeThrow; both pipeline paths now catch and answer it as the call's value, matching finished
- The ladder splits by API to match Node exactly: finished keeps the isNodeStream clause (null now included), while pipeline stages render makeAsyncIterable's "body" clause — the same text in source, middle, and destination positions — for numbers, booleans, null, and pair-less records
- Shapes Node accepts but scriptc can't lower yet never take a throw Node never throws: strings and arrays (iterables) and records carrying a readable/writable member (the duplex-pair form) keep the pointed fence, whose hint now names those inputs
- tests/corpus/2632-stream-pipeline-arg-ladders.cjs pins the ladder in all three positions plus finished(null) and a live pipe, byte-identical through both backends in the plain and sanitized lanes