php-in-php: StringCaseCompare NestedJIT → JitVmHelperLink::ensureCompiled (#23862)

[PurHur]

Summary

• Replace hand-rolled NestedJitCompileScope::run + parseAndCompile in StringCaseCompare.php with JitVmHelperLink::ensureCompiled + lookupCompiled (peer StringStrtotime php-in-php: StringStrtotime NestedJIT → JitVmHelperLink::ensureCompiled (#1492) #23832).
• Add shrink guard in StrcasecmpRuntimeShrinkTest asserting no hand-rolled NestedJIT remains.
• Net −12 lines in StringCaseCompare.php (removed duplicate compile loop; helper-runtime cache path now shared).

Closes #23862

php-src reference

• ext/standard/string.c — PHP_FUNCTION(strcasecmp|strncasecmp)

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
vendor/bin/phpunit test/unit/StrcasecmpRuntimeShrinkTest.php --filter testStringCaseCompareUsesJitVmHelperLink
make bootstrap-selfhost-vm-driver-execute-probe
'

Output:

OK (1 test, 8 assertions)
bootstrap-selfhost-vm-driver-execute-probe: OK /compiler/build/selfhost-lib-spine-smoke

Also ran (pre-change baseline):

make bootstrap-selfhost-link  → OK /compiler/build/selfhost
php script/bootstrap-inventory.php --check  → inventory green

Not covered

• Full north-star5-verify-fast (change is isolated JIT helper link; VM driver probe green)
• Differential sweep (no argument/call lowering change — compile-path refactor only)

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