JIT: stub external Class::method for self-host bundle (Fixes #579)

[PurHur]

Summary

Implements Option A from #579: at JIT compile time, undefined Class::method and static Class::method dispatches are bound to JIT\Call\ExternalMethod stubs instead of aborting.

• Instance/static calls: Context::resolveFunctionProxy() lazily registers ExternalMethod, which records the proxy name in Context::$externalMethodStubs and emits LLVM that returns null (__value__writeNull). No Zend/vendor code runs in the native binary.
• External classes: Object_::lookup() registers unknown class names (e.g. ReflectionClass, PhpParser\NodeTraverser) so new and instanceof lowering can proceed.
• Static calls: TYPE_STATICCALL_INIT now uses the same proxy path as instance calls (master already called resolveFunctionProxy for instance calls but the implementation was missing).

Progress (Docker php-compiler:22.04-dev)

Before	After
Call to undefined method phpparser\nodetraverser::addvisitor (and similar)	JIT proceeds past vendor method calls
—	Next blocker on bundled compile: IteratorHelper / __value__ literals (separate issues)

php bin/compile.php -l test/bootstrap-aot/external_method_stub.php passes.

Stubbed vs implemented

API	Treatment
Any bundled Class::method without a compiled TYPE_DECLARE_METHOD body	Stub → returns null
phpcompiler\… methods with CFG in the bundle	Implemented (existing Call\Native proxies)
Builtin/internal proxies (is_string, VM context, …)	Implemented (unchanged)

Runtime introspection: after a successful link, JIT\Context::$externalMethodStubs lists proxy names touched during codegen (e.g. phpparser\nodetraverser::addvisitor).

Test plan

• phpunit test/unit/JitExternalMethodStubTest.php (Docker)
• php bin/compile.php -l test/bootstrap-aot/external_method_stub.php
• ./script/bootstrap-selfhost-link.sh (still fails past IteratorHelper; progress gate only)

Fixes #579

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