WIP: Add parallel test execution to jcpan (-j flag)

[fglock]

Summary

Add parallel test execution to jcpan and fix the Module::Build dependency chain for DateTime::Format::ICal.

Parallel test execution (-j N flag)

• jcpan/jcpan.bat: Parse -j N, export HARNESS_OPTIONS=jN
• TAP::Parser::Iterator::Process: Enable IO::Select on pipe handles in the non-fork fallback path, allowing the TAP::Parser::Multiplexer to use select()-based I/O multiplexing

jcpan -j 4 -t Module::Name    # 4 parallel test jobs
jcpan -t Module::Name          # sequential (default, unchanged)

Benchmark (Path::Tiny, 30 test files): 36s sequential -> 18.5s with -j 4 (~2x speedup)

Module::Build chained shebang fix

• CPAN::Distribution: ./Build relies on chained shebangs which don't work when jperl is a bash wrapper script. Fixed _build_command() and the install path to use $^X Build when archname contains java.

Module::Build auto-pureperl

• Module::Build::Base: Auto-enable pureperl_only when a module declares allow_pureperl and no C compiler is available. Modules like Params::Validate now build their pure-Perl backend instead of failing on XS compilation.

Devel::Peek stub

• Minimal stub providing SvREFCNT() (returns 1 — JVM uses tracing GC) and Dump(). Needed as test_requires for Params::Validate.

Result

$ ./jcpan -j 4 -t DateTime::Format::ICal
t/00load.t ........ ok
t/01datetime.t .... ok
t/02duration.t .... ok
t/03period.t ...... ok
t/04recurrence.t .. ok
All tests successful.
Files=5, Tests=134
Result: PASS

Files changed

File	Change
jcpan	Parse -j N, export HARNESS_OPTIONS
jcpan.bat	Same for Windows
TAP::Parser::Iterator::Process	IO::Select on pipe handles
CPAN::Distribution	$^X Build instead of ./Build on PerlOnJava
Module::Build::Base	Auto-pureperl when no compiler
Devel::Peek	New stub module
dev/modules/datetime_format_ical.md	Tracking doc

Test plan

• make passes (all unit tests)
• jprove -j 4 works with unit tests
• jcpan -j 4 -t Path::Tiny same results as sequential
• jcpan -j 4 -t DateTime::Format::ICal — all 134 tests pass
• jcpan -f -i Params::Validate installs successfully
• jcpan -f -i DateTime::Set installs and tests pass
• Test on Windows

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