feat(playground): show the sandbox prelude output collapsed

[pavlof01]

Under --print-bytecode the V8 tab printed five generated bytecode blocks for a one-line snippet; four of them belonged to the sandbox lockdown shim the engine service loads ahead of the snippet.

• engine-runtime: whenever any prelude loads (the lockdown shim for d8 and jsc, the console shim for jsc), a final prelude-end.js prints a marker line. stdout is returned from the line after it, and everything before it comes back as meta.preludeStdout. Output without a marker is returned untouched; Hermes and SpiderMonkey have no preludes and are unaffected.
• frontend: the playground shows preludeStdout as a collapsed "sandbox prelude · N lines" row above the engine output; clicking it expands the prelude with the same highlighting and diff support. The snippet's own bytecode is what the pane opens on.

Verified on the dev stack: for function f(x){ return x + 1 } f(41); the pane shows the snippet's two blocks, and the collapsed row holds the 150-line prelude. For jsc one End: undefined line from prelude-end.js itself remains in stdout and the marker text shows up in the -d disassembly on stderr; that is left for a follow-up.

🤖 Generated with Claude Code