Link task functions in destination runtime

[realFlowControl]

Summary

• Link direct task calls against the destination runtime's function table.
• Execute destination-sensitive user-function calls through copied VM opcodes so function JIT cannot retain a submitter-runtime target.
• Resolve unavailable functions when invoked, preserving task-local includes and catchable undefined-function errors.
• Keep unaffected shared worker opcodes JIT-capable and verify this with a focused tracing-JIT test using opcache.protect_memory=0.
• Document Runtime bootstrap as the preferred way to provide task dependencies.

Broad tracing-JIT CI remains separate in #394 pending the upstream OPcache fix in php/php-src#23081.

Testing

• Full PHP 8.6 ZTS suite under vanilla, OPcache, and function JIT: zero failures.
• Focused tracing-JIT tests with opcache.protect_memory=0: five passes.