Fix :Generator without yield rejecting scalar return (#10333)

[PurHur]

Summary

• Stop auto-marking every : Generator function as a generator; only callables with yield/yield from in source (AST marker) or compiled opcodes are generators (Language: Generator::getReturn() — return value from generator (Zend zend_generators.c parity) #3350 / Language: Generator::getReturn() on return-before-yield — must return value not Exception (Zend/zend_generators.c) #9007 preserved).
• Enforce declared object return types on non-generator callables so function g(): Generator { return 1; } throws TypeError at the call site, matching Zend/php-src.
• Add GeneratorYieldSourceMarker AST pass so unreachable yield after return still marks the callable as a generator when php-cfg DCE removes the yield opcode.

php-src reference: Zend/zend_execute.c (return-type validation), Zend/zend_generators.c (generator creation requires yield in body)

PHP implementation: lib/Ast/GeneratorYieldSourceMarker.php, lib/Compiler.php, lib/VM.php, lib/VM/TypeCheck.php

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro/maintainer_gap_generator_return_scalar.php 2>&1 | head -3
php bin/vm.php test/repro/maintainer_gap_generator_return_scalar.php 2>&1 | head -5
./vendor/bin/phpunit --filter "generator_get_return|generator_return_type_scalar"
'

Output:

PHP Fatal error:  Uncaught TypeError: g(): Return value must be of type Generator, int returned ...  # Zend
PHP Fatal error:  Uncaught TypeError: g(): Return value must be of type Generator, int returned ...  # VM
OK (10 tests, 10 assertions)

Closes #10333

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