fix: retry stalled Bun installs

[merryman]

Summary

• detect Bun installs that produce no output for 120 seconds
• terminate a stalled attempt and retry once using the partially warmed cache and work directory
• force-kill an unresponsive attempt after a short grace period
• preserve live output in verbose mode and add a regression test with a simulated stalled Bun process

Root cause

The installer waited indefinitely for the spawned Bun process while suppressing its normal output. When Bun intermittently stalled in its cache or download path, the install had no timeout or recovery path even though manually restarting the same install often succeeded.

Impact

make install and make clean-install can now recover automatically from a transient Bun stall. If both attempts stall, the existing Flatn fallback still handles the failure. Slow environments can override the default with LIVELY_BUN_INSTALL_STALL_TIMEOUT_MS.

Validation

• git diff --check origin/main...HEAD
• node --check flatn/bun-install.js
• simulated first-attempt stall followed by a successful retry
• full legacy Flatn suite was not run because mocha-es6 is unavailable in this checkout