PR #161 review: removing ReadRegular's type check on the opened handle
stayed green, because the race test's FIFOs had no writer and read EOF
at once; with a writer holding the FIFO open, search hung after 16
iterations. The open-and-check step is split into readOpened, and
TestReadOpenedRefusesAFIFOHeldOpenByAWriter reaches it past the Lstat
with a writer open: it must return ErrNotRegular promptly (the flip is
red).
Also unit tests for ReadRegular (symlink, escape), Rel (inside incl.
..foo, escapes) and WalkRegular (regular files only), and the hard-link
residual documented where it is accepted, as in api/rawroot.go.
Refs #157
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
