fix: open on broken symlink returns ELOOP instead of confess

[Koan-Bot]

Summary

• open() on a broken/circular symlink now returns false with $! = ELOOP instead of dying via confess()
• sysopen() with O_NOFOLLOW on a symlink now uses portable ELOOP constant instead of hardcoded $! = 40 (which is EMSGSIZE on macOS, not ELOOP)

What changed

__open: The broken symlink check (confess() if $abs_path eq BROKEN_SYMLINK) was a fatal error with no message. Real open() on a broken symlink returns false and sets $! to ELOOP. Now handles both BROKEN_SYMLINK and CIRCULAR_SYMLINK correctly.

__sysopen: The O_NOFOLLOW handler used $! = 40 — a hardcoded errno value that only maps to ELOOP on Linux. On macOS/FreeBSD, errno 40 is EMSGSIZE. Now uses the ELOOP constant from Errno (already imported).

Test plan

• New test in t/open.t: opens a broken symlink, verifies returns false with $! == ELOOP
• New test in t/sysopen.t: sysopen with O_NOFOLLOW on a symlink, verifies returns false with $! == ELOOP

🤖 Generated with Claude Code

[atoomic]

View failure from CI

Test Summary Report
-------------------
t/open.t                          (Wstat: 512 (exited 2) Tests: 17 Failed: 0)
  Non-zero exit status: 2
  Parse errors: No plan found in TAP output
t/sysopen.t                       (Wstat: 65280 (exited 255) Tests: 75 Failed: 0)
  Non-zero exit status: 255
  Parse errors: No plan found in TAP output
Files=43, Tests=514,  5 wallclock secs ( 0.10 usr  0.05 sys +  4.14 cusr  1.32 csys =  5.61 CPU)
Result: FAIL
Failed 2/43 test programs. 0/514 subtests failed.

[atoomic]

@Koan-Bot rebase

[Koan-Bot]

CI fix — force-pushed 4ae8b6b

Root cause: Both tests had swapped symlink() constructor arguments ($readlink, $file order). This caused:

• t/open.t: Created file mock at /target_file, then symlink also at /target_file → confess("already being mocked") (exit 2)
• t/sysopen.t: Same pattern — symlink at /some_target then file at /some_target → confess (exit 255)

Fixes:

1. t/open.t: Removed unnecessary target file mock. A true broken symlink = symlink to a path with no mock at all. _find_file_or_fh then correctly returns BROKEN_SYMLINK
2. t/sysopen.t: Fixed argument order — create file first, then symlink pointing to it. __sysopen checks is_link on the symlink mock → O_NOFOLLOW triggers ELOOP
3. __open: Changed BROKEN_SYMLINK errno from ELOOP to ENOENT — matches real kernel behavior (ELOOP is only for circular symlinks; broken symlinks get ENOENT)