Stream playground runs with a Stop button, dropping the 5 s hard kill

[baijum]

Closes #32.

What

Replaces the playground/tour's blunt 5 s hard-timeout (which terminate()d the Web Worker, killing legitimately long-running constant-space programs and discarding any output already produced) with cooperative, streaming execution built on the new bounded-step WASM entry point (kaappi#2283, merged in kaappi#2284).

• Pump loop — playground-worker.js runs kaappi_step_run(budget) a chunk at a time, yielding to the event loop between chunks.
• Streaming output — stdout/stderr flush to the page as produced (batched per chunk; per-line postMessage floods the main thread on a fast print loop) and are capped per stream so an unbounded loop can't balloon the DOM. The program keeps running regardless.
• Stop button — a real cooperative stop (kaappi_step_stop() → the VM unwinds at its safepoint, running dynamic-wind after-thunks), replacing terminate-and-respawn. Output already produced is preserved.
• Wall-clock backstop — a generous 30 s cooperative limit instead of a 5 s kill; kp-runner.mjs keeps a hard-kill timer only as a last resort for a wedged/blocking worker.

Safe to merge before a core release

The released WASM the site currently fetches (v0.22.3) does not yet export the step entry point. The worker feature-detects the exports:

• exports present → stepped path (streaming + Stop, above);
• exports absent → batch path — the classic blocking WASI _start, identical to today's behavior.

So this deploys safely against the current WASM (batch) and upgrades to streaming automatically once update-wasm syncs a kaappi release that ships the exports. No coordination window.

Testing

Verified in-browser (mkdocs serve) against a zig build wasm binary from core main (step exports present):

• (display …) / result echo / error-to-stderr all match the batch output;
• a finite 0…999 loop streams and completes (Done);
• the classic constant-space non-terminator (let loop ((i 0)) (display i)(newline)(loop (+ i 1))) streams ticks live and stops cleanly on the Stop button (no hang, output capped with a truncation notice, buttons reset);
• same on the tour page.

Batch fallback verified against the released v0.22.3 WASM (no step exports): feature-detect correctly selects batch and output is identical.

mkdocs build --strict passes.

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