Self-host M3: lower emit-TU spine before bridge

[PurHur]

Summary

• Register PHPCompiler\Compiler::compileEmitSmoke native decl during M3 emit-TU spine pre-lowering.
• Flush pending lowering (runQueue) before emitting the M3 emit-TU native {main} bridge, avoiding binding to null-returning stubs.

Why

Emit-TU binaries were executing the native bridge against stubbed Runtime/Compiler symbols, producing parseAndCompile returned null and forcing emit_path=zend partial in self-host probes.

This matches the root cause described in #2442 (sub-issue #2512).

Verification (Docker)

./script/docker-exec.sh -- bash -lc '
  composer install --no-interaction --prefer-dist --ignore-platform-reqs >/dev/null \
  && ./script/apply-patches.sh >/dev/null \
  && source script/php-env.sh \
  && BOOTSTRAP_M3_LINK_COMPILE_DRIVER=1 \
     BOOTSTRAP_M3_COMPILE_DRIVER_REAL_LOWERING=1 \
     BOOTSTRAP_M3_RUNTIME_COMPILE=1 \
     ./script/bootstrap-selfhost-helloworld-probe.sh
'

• Result change: the previous parseAndCompile returned null failure is eliminated.
• Current blocker after this change: emit-TU helper still hits the next known failure (LLVM 9 emit TU runtime init / global ctor), tracked separately (Self-host M3: VM\Context native object allocation — complete C-floor for Runtime::initVmContext #2513 / Self-host M3: native emit-TU binaries run without startup crash #2442 follow-ups).

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