Two unrelated issues blocked HTTP::Client::Parallel:

1. POSIX.pm did not export the termios constants nor the _SC_*
   sysconf() name constants used by POE::Wheel::Run (loaded
   transitively from HTTP::Client::Parallel via POE). The Java side
   already implemented _const_ECHO, _const_TCSANOW, etc., and
   POSIX.pm wired them into POSIX::ECHO, POSIX::TCSANOW, ... but
   they were missing from @EXPORT_OK so `use POSIX qw(ECHO ...)` died
   with "ECHO is not exported by the POSIX module". Add the termios
   and _SC_* names to @EXPORT_OK and provide a small sysconf() stub
   that returns sensible defaults (e.g. _SC_OPEN_MAX => 1024).

2. The 1-argument form `open FILEHANDLE` (used by Module::Install::DSL,
   which LWP::Online ships) crashed with "Modification of a read-only
   value" when the filehandle was given as a numeric literal such as
   `open 0`. The runtime tried to call set() on the readonly arg and
   was also picking the filename from $_ instead of from the global
   scalar of the same name as the filehandle. Rewrite the 1-arg open
   path to derive the filehandle name from the value (covering glob
   refs, bareword strings, and numeric literals like 0), look up the
   filename in $main::<name>, and only assign back to args[0] when it
   is a writable scalar.

With both fixes, `jcpan -t HTTP::Client::Parallel` and its
LWP::Online dependency build and pass their unit tests.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>