Fix wave 12 bootstrap AOT resolver/superglobal/string regressions (#1086)

[PurHur]

Summary

• Restore bootstrap-aot correctness when PHP_COMPILER_SELFHOST_AOT=1 is inherited from the environment (CI/dev shells): compile bootstrap fixtures with PHP_COMPILER_SELFHOST_AOT=0 and keep trim/urlencode on the real JIT path via SelfHostBuiltinPolicy.
• Return owned __string__* copies from trim/urlencode JIT (__string__separate) so nested string compares/strlen no longer segfault on module-string constants.
• Keep self-host probe green by leaving ConstStringFolder stubbed under self-host AOT (probe ICmp); resolver/superglobal bootstrap fixtures use real lowering via the bootstrap-aot compile gate.

Root cause

Commit 4404c25 (#1097) widened self-host stubbing (Superglobals::*, IncludePathResolver::resolve) and trim/urlencode were absent from SelfHostBuiltinPolicy, so with inherited PHP_COMPILER_SELFHOST_AOT=1 bootstrap-aot-link compiled stubbed/no-op builtins. String builtins returning module __string__* constants without __string__separate also broke consumers (strlen, !==).

Before / after stdout (bootstrap-aot fixtures)

Fixture	Before (master)	After
include_path_resolver_smoke.php	0\n1\n1	1\n1\n1
superglobal_name_check.php	00	10
stdlib_string_trim.php	(empty / crash)	1\n
stdlib_string_urlencode.php	(empty / crash)	1\n

Test plan

• PHP_COMPILER_SELFHOST_AOT=0 bin/compile.php on all 4 fixtures → stdout matches Zend
• make bootstrap-selfhost-probe (probe still green)
• ./script/bootstrap-aot-link.sh for the 4 targets (via PHP_COMPILER_SELFHOST_AOT=0 compile prefix)

Partial #1086 — squash when green.

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