[PurHur]

Summary

• Extend AotEmitFastExit::exitAfterSuccessfulSelfhostEmit() to honor PHP_COMPILER_SPINE_CHUNK=1, not only test/selfhost/ paths and PHP_COMPILER_SELFHOST_AOT=1.
• Belt-and-suspenders in bin/compile.php: call fast-exit after successful emit when ExternalMethodBind::spineChunkMode() is on.
• Add unit assertions in SelfhostAotFastExitTest.

Why: Spine split-TU hub/consumer chunks (script/spine-chunk-probe.sh, bootstrap-gen0-chunk-emit.sh) compile build/micro/chunk/*.php with SPINE_CHUNK=1. bin/compile.php treats those as user-script AOT and clears SELFHOST_AOT, so LLVM teardown after a successful -o link could spin 15+ min (#31726) even though the binary was already runnable. This blocked Phase B/C hub manifest export timing out at 600–900s wall (compile + hang).

Not in scope: Core hub JIT compile of spine-chunk-core-requires.txt still exceeds 10 min on this host — separate compile-time work, not post-emit hang.

Closes #36155 (follow-up slice).

Test plan

./script/phpunit.sh --filter SelfhostAotFastExitTest
# OK (9 tests, 27 assertions)

make dev-verify-fast
# OK (72s) — aot-smoke 8/8, VM differential tier-0 15/15

./script/docker-exec.sh -- bash -lc 'echo "<?php echo 1;" > /tmp/tiny.php && env PHP_COMPILER_SPINE_CHUNK=1 php bin/compile.php -o /tmp/tiny.bin /tmp/tiny.php'
# EXIT:0 ~6s, /tmp/tiny.bin non-empty

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