fix: download p2pd on first use, using got

[tabcat]

The postinstall download is best-effort: npm v12 makes install scripts opt-in by default, and --ignore-scripts, pnpm and hardened CI already skip them, so the p2pd binary can silently go missing. Separately, the download used the global fetch, whose response body drain stalls indefinitely on some platforms (e.g. GitHub's macos-26 runners).

• Keep the postinstall as a fast path, but download on first use as a fallback: path() runs the installer in a child process when the binary isn't present, so a skipped script heals itself. Pass path({ autoDownload: false }) to resolve only an already-installed binary.
• Download with got instead of the global fetch. got reads the body reliably and handles retries/backoff itself (drops the now-unused p-retry, delay, browser-readablestream-to-it and it-to-buffer).
• bin/p2pd ships as a JS stub the installer replaces, so path() treats that stub as not-installed and downloads on first call.

Same approach as ipfs/npm-kubo#83.

[tabcat]

Note on retry handling: I dropped the custom retries/retryDelay options (and the undocumented RETRIES/RETRY_DELAY env vars) and switched to plain got(url).buffer(), so downloads now use got's built-in retry and backoff defaults. This keeps it in line with how npm-kubo does it (ipfs/npm-kubo#83).

Happy to restore the knobs if you'd prefer to keep them (got's retry.calculateDelay can reproduce the old fixed delay), but matching kubo felt cleaner.