Fix: SPINE_CHUNK sockets compile past SIGSEGV/verify (#24429)

[PurHur]

Summary

• Restore TYPE_STRING guard on Helper's compileTimeString IDENTICAL fast-path so VALUE-boxed string literals are not passed to VmStringCompare::identical as native __string__* (was SIGSEGV / structGep kind=10 under PHP_COMPILER_SPINE_CHUNK=1 on ext/sockets/SocketsLibcThinAbi.php).
• Coerce typed __object__* returns via JitValueBox::valuePtrFromVariable instead of store __value__* into alloca __value__ (module verify failure in SocketsLibcThinAbi::ffi() after the string-compare crash was cleared).
• Compile-only repro: test/repro/issue_24429_static_nullable_object_return.php.

Closes #24429 — partial: removes the sockets SIGSEGV/verify wall for SocketsLibcThinAbi; full ext/sockets chunk still OOMs at ≥4G (next capacity wall), and unbound ExternalMethod stubs remain (#579).

php-src / prior art

• Guard restores the intent of unmerged 1a719cbe6 (bootstrap-aot-link: VALUE-box concat + ordered float compare lowering #8555 getenv/compileTimeString narrowing).
• Return coerce uses existing JitValueBox::valuePtrFromVariable (already handles KIND_VALUE __value__*).

Test plan

# Was: structGep / SIGSEGV; now compiles
PHP_COMPILER_LLVM_ASSERT=1 PHP_COMPILER_SPINE_CHUNK=1 \
  php bin/compile.php -o /tmp/repro24429.bin \
  test/repro/issue_24429_static_nullable_object_return.php
# → RC 0

# Prior fallthrough still OK
php bin/compile.php -o /tmp/d1.bin test/repro/issue_24429_static_call_fallthrough.php  # RC 2
PHP_COMPILER_SPINE_CHUNK=1 PHP_COMPILER_REPORT_EXTERNAL_STUBS=1 \
  php bin/compile.php -o /tmp/d2.bin test/repro/issue_24429_static_call_fallthrough.php  # RC 0

php vendor/bin/phpunit --filter ExternalMethodBindTest  # 8 OK

# Full sockets chunk: no longer rc=139; default 1536M / 4G still OOM (capacity, not crash)
script/spine-chunk-probe.sh --ext=sockets

Not covered

• Full sockets/spl chunk link within memory budget
• Typed per-chunk manifests / real ExternalMethod binding (Spine split-TU is blocked on cross-chunk binding: no spine chunk compiles in isolation today #24429 remaining)
• Runtime of typed static ?object returns (pre-existing abort on master; separate)

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