v0.15.2 — self-update no longer fails with Text file busy

Fixed

• self-update no longer aborts with Text file busy when verifying the
  staged binary. The installer held its own O_RDWR staging handle open
  while trying to execute the staged file, and Linux refuses execve on any
  file with an open write descriptor (ETXTBSY) — so every install failed
  with staged binary cannot start on this host: Text file busy, left the
  previous binary untouched, and suggested retrying, which could never help.
  The write handle is now dropped and the staged file reopened read-only
  before the probe runs. Production always used the real exec-based probe,
  but the tests stubbed it out, which is why the bug survived review; a new
  regression test mirrors the ETXTBSY precondition by rejecting the install
  while any write descriptor still points at the staged file.