fix(runtime): reject zero-progress JS stream writes

[vicmcorrea]

Problem and fix

The JS runtime's standard-stream writer retries without advancing when fs.writeSync returns zero for nonempty data; repeated zero returns make it spin indefinitely. Reject that result through its existing short-write failure path (exit 1), using the same small guard as the CLI writer in #1691. This extends the zero-progress rule already applied to File.write in #1446.

Blast radius

Five replacement lines in bend2/comp.ts's JS io_out, used by stdout/stderr effects and emitted result output, plus tests/io/stream_write_zero.{bend,js}. Successful writes, positive short writes, EINTR/EAGAIN retries and EPIPE→SIGPIPE behavior retain their existing paths. A nonempty zero-byte host return now fails immediately instead of retrying. Native C and the separate CLI writer are unchanged.

Regression evidence

The JS-only fixture serializes the runtime's actual io_out into bounded children and injects zero returns for stdout and stderr. Source execution and emitted Bun/Node print 1 on main f5cee564 (fails expected 0) and 0 with the fix. Each child has a two-second deadline; the broken fixture stops after its first failed child.

Checks

• Repository gate: 54/54 with CI-pinned ttok 0.3; comp.ts is 65,541/66,000 tokens. git diff --check and GitHub’s repo-gate check pass.
• New fixture in source, --checkup imports and emitted Bun/Node. Eight fixtures, including the new regression, pass their applicable local lanes (30 checks).
• Actual emitted-program controls cover ordinary output, bytewise UTF-8 short writes, EINTR/EAGAIN, empty output and zero after partial progress. Real closed-reader pipes preserve SIGPIPE under Bun and Node.
• Emitted C for two stdout/pure-result samples is byte-identical before/after.

Validation limits

Zero returns are deliberately injected; no ordinary OS occurrence or prevalence is established. Checked locally on macOS arm64, Bun 1.3.11 and Node 26.8.2. Full cluster tests, Linux runtime tests and GPU checks were not run; GitHub CI ran the repository gate on Linux. The existing file-write and EPIPE foreign fixtures require Bun FFI, so their Node lanes were skipped; separate Node controls cover the changed writer and real EPIPE. C text comparison is limited to two samples; native binaries and a full TypeScript check were not run.