fix(core): make CastObjectHook fall through to default engine behaviour safely

[lisachenko]

Fixes #153.

The bugs

Two defects made the documented fall-through pattern — $hook->proceed(); return $hook->getResult(); — unusable for cast types the default handler cannot satisfy:

1. getResult() read uninitialized memory. The engine hands cast_object an uninitialized retval slot, and zend_std_cast_object_tostring returns FAILURE for numeric casts without writing it (the engine caller is what warns and substitutes 1). getResult() dereferenced the slot regardless, running the by-ref ReferenceEntry machinery over garbage — observably unsetting local variables in the calling frames ($result in handle() itself became undefined after being assigned) and emitting spurious "Undefined variable" warnings.
2. handle() always reported Core::SUCCESS, so a FAILURE from the original handler could never propagate. Whatever the user handler returned — including an accidental null — was silently installed as the cast result, and the engine caller's default behaviour (warning + substitute value for numeric casts, engine Error for string casts) was unreachable.

The fix

• proceed() records its status.
• getResult() refuses to read the slot unless the last proceed() succeeded, returning null instead of dereferencing scratch memory.
• handle() propagates FAILURE to the engine when the user handler fell through (returned null after a failed proceed()). The failure is surfaced by the engine's own caller after the FFI callback has returned, so even the throwing default paths (string casts) stay safe.

Together this makes the naive fall-through behave exactly like an uninstalled handler for every cast type.

The regression test

testCastObjectHandlerFallsThroughToEngineDefault installs the naive fall-through handler and asserts:

• boolean conversion yields the default handler's true (via getResult() after a successful proceed);
• (int) yields 1 with the engine's own diagnostic Object of class ... could not be converted to int and — by capturing every PHP diagnostic into a strict assertSame — no "Undefined variable" corruption noise.

Against the unfixed hook the test fails with exactly the reported symptom (captured warning is Undefined variable $result instead of the engine diagnostic); with the fix it passes.

Verified: full suite green on PHP 8.4.19 NTS (409 tests), PHPStan level max clean, PER-CS2.0 clean.

Note: this PR touches the same file as #155 in non-overlapping regions; both are independent of each other and either merge order works.

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